update readme with usage instructions

README.md

@@ -1,3 +1,27 @@
 # Natchez-Smithy4s
 
-Utilities for integration between [Natchez](https://github.com/typelevel/natchez) and [Smithy4s](https://disneystreaming.github.io/smithy4s/).
+Utilities for integration between [Natchez](https://github.com/typelevel/natchez) and [Smithy4s](https://disneystreaming.github.io/smithy4s/).
+
+## Usage
+
+Add the library to your build:
+
+```scala
+"com.dwolla" %% "natchez-smithy4s" % {version} 
+```
+
+Then create a file to [annotate your shapes](https://disneystreaming.github.io/smithy4s/docs/guides/model-preprocessing/#note-on-third-party-models):
+
+```smithy
+$version: "2.0"
+namespace com.dwolla.example.smithy
+
+use com.dwolla.tracing.smithy#traceable
+
+apply CipherText @traceable
+apply PlainText @traceable(redacted: "redacted plaintext value")
+```
+
+The `@traceable` trait can be applied without any modifier, in which case a `natchez.TraceableValue` instance will be generated that includes the actual value of the field.
+
+If the `@traceable` trait is used with a `redacted` modifier, the `TraceableValue` instance will emit the passed string and not reference the actual value of the field in any way.