Document using DOCC_EXEC in SwiftPM

Author: natikgadzhi

README.md

@@ -144,7 +144,22 @@ The next time you invoke a documentation build with the "Build Documentation"
 button in Xcode's Product menu, your custom `docc` will be used for the build.
 You can confirm that your custom `docc` is being used by opening the latest build
 log in Xcode's report navigator and expanding the "Compile documentation" step.
-  
+
+### Invoking `docc` from Swift Package Manager
+
+You can also test a locally built version of Swift-DocC using Swift Package Manager
+CLI. SwiftPM will try to read `DOCC_EXEC` environment variable value, and use
+the path you provded if it's set.
+
+  1. In your test `Package.swift`, add a dependency on `swift-docc-plugin`.
+     [`swift-docc-plugin`](https://github.com/apple-swift-docc-plugin) is a 
+     Swift Package Manager plugin that invokes `docc` for you, see details in 
+     `swift-docc-plugin`.
+  2. Set `DOCC_EXEC` environment variable and run documentation generation
+     command like this:
+     
+        `DOCC_EXEC=/path/to/docc swift package generate-documentation`
+
 ## Using `docc` to build and preview documentation
 
 The preferred way of building documentation for your Swift package is by using