Document use of the build plugin

Motivation:

To ease use of the `grpc-swift-protobuf` Swift Package Manager build
plugin.

Modifications:

Documentation added.

Result:

Greater understanding?

Sources/GRPCCore/Documentation.docc/Articles/Generating-stubs.md

@@ -4,8 +4,6 @@ Learn how to generate stubs for gRPC Swift from a service defined using the Prot
 
 ## Overview
 
-### Using protoc
-
 If you've used Protocol Buffers before then generating gRPC Swift stubs should be simple. If you're
 unfamiliar with Protocol Buffers then you should get comfortable with the concepts before
 continuing; the [Protocol Buffers website](https://protobuf.dev/) is a great place to start.
@@ -16,7 +14,17 @@ The [`grpc-swift-protobuf`](https://github.com/grpc/grpc-swift-protobuf) package
 > `protoc-gen-grpc-swift` only generates gRPC stubs, it doesn't generate messages. You must use
 > `protoc-gen-swift` to generate messages in addition to gRPC Stubs.
 
-To generate gRPC stubs for your `.proto` files you must run the `protoc` command with
+The protoc plugin can be used from the command line directly, passed to `protoc`, or 
+you can make use of a convenience which adds the stub generation to the Swift build graph.
+The automatic gRPC Swift stub generation makes use of a [Swift Package Manager build plugin](
+https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/Plugins.md) to use the 
+`.proto` files as inputs to the build graph, input them into `protoc` using `protoc-gen-grpc-swift` 
+and `protoc-gen-swift` as needed, and make the resulting gRPC Swift stubs available to code 
+against without committing them as source. The build plugin may be invoked either from the command line or from Xcode.
+
+### Using protoc
+
+To generate gRPC stubs for your `.proto` files directly you must run the `protoc` command with
 the `--grpc-swift_out=<DIRECTORY>` option:
 
 ```console
@@ -69,7 +77,7 @@ allows you to specify a mapping from `.proto` files to the Swift module they are
 allows the code generator to add appropriate imports to your generated stubs. This is described in
 more detail in the [SwiftProtobuf documentation](https://github.com/apple/swift-protobuf/blob/main/Documentation/PLUGIN.md).
 
-#### Building the plugin
+#### Building the protoc plugin
 
 > The version of `protoc-gen-grpc-swift` you use mustn't be newer than the version of
 > the `grpc-swift-protobuf` you're using.
@@ -83,3 +91,83 @@ swift build --product protoc-gen-grpc-swift
 
 This command will build the plugin into `.build/debug` directory. You can get the full path using
 `swift build --show-bin-path`.
+
+## Using the build plugin
+
+The build plugin (`GRPCProtobufGenerator`) is a great choice for convenient dynamic code generation, however it does come with some limitations.
+Because it generates the gRPC Swift stubs as part of the build it has the requirement that `protoc` must be guaranteed
+to be available at compile time. Also because of a limitation of Swift Package Manager build plugins, the plugin
+will only be invoked when applied to the source contained in a leaf package, so it is not useful for generating code for
+library authors.
+
+The build plugin will detect `.proto` files in the source tree and perform one invocation of `protoc` for each file 
+(caching results and performing the generation as necessary).
+
+### Adoption
+Swift Package Manager build plugins must be adopted on a per-target basis, you can do this by modifying your 
+package manifest (`Package.swift` file). You will need to declare the `grpc-swift-protobuf` package as a package
+dependency and then add the plugin to any desired targets.
+
+For example, to make use of the plugin for generating gRPC Swift stubs as part of the 
+`plugin-adopter` target:
+```swift
+targets: [
+   .executableTarget(
+     name: "plugin-adopter",
+     dependencies: [
+       // ...
+     ],
+     plugins: [
+       .plugin(name: "GRPCProtobufGenerator", package: "grpc-swift-protobuf")
+     ]
+   )
+ ]
+```
+Once this is done you need to ensure that that the `.proto` files to be used for generation 
+are included in the target's source directory (below relevant the `Source` directory) and that you have 
+defined at least one configuration file.
+
+### Configuration
+
+The build plugin requires a configuration file to be present in a directory which encloses all `.proto` files 
+(in the same directory or a parent).
+Configuration files are JSON which tells the build plugin about the options which will be used in the
+invocations of `protoc`. Configuration files must be named `grpc-swift-proto-generator-config.json`
+and have the following format:
+```json
+{
+  "generate": {
+    "clients": true,
+    "servers": true,
+    "messages": true,
+  },
+  "generatedSource": {
+    "accessLevelOnImports": false,
+    "accessLevel": "internal",
+  }
+  "protoc": {
+    "executablePath": "/opt/homebrew/bin/protoc"
+    "importPaths": [
+      "../directory_1",
+    ],
+  },
+}
+```
+
+The options do not need to be specified and each have default values.
+
+| Name                                   | Possible Values                            | Default                              | Description                                              |
+|----------------------------------------|--------------------------------------------|--------------------------------------|----------------------------------------------------------|
+| `generate.servers`                     | `true`, `false`                            | `True`                               | Generate server stubs                                    |
+| `generate.clients`                     | `true`, `false`                            | `True`                               | Generate client stubs                                    |
+| `generate.messages`                    | `true`, `false`                            | `True`                               | Generate message stubs                                   |
+| `generatedSource.accessLevelOnImports` | `true`, `false`                            | `false`                              | Whether imports should have explicit access levels       |
+| `generatedSource.accessLevel`          | `public`, `package`, `internal`            | `internal`                           | Access level for generated stubs                         |
+| `protoc.executablePath`                | N/A                                        | N/A (attempted discovery)            | Path to the `protoc` executable                          |
+| `protoc.importPaths`                   | N/A                                        | Directory containing the config file | Access level for generated stubs                         |
+
+Many of these map to `protoc-gen-grpc-swift` and `protoc-gen-swift` options.
+
+If you require greater flexibility you may specify more than one configuration file.
+Configuration files apply to all `.proto` files equal to or below it in the file hierarchy. A configuration file
+lower in the file hierarchy supersedes one above it.