Add README

Author: kubukoz

README.md

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+# smithy-trait-codegen-scala
+
+Generate Java classes (with builders, serialization to/from `Node`, trait providers) for Smithy traits, for usage in Scala projects.
+
+## Usage
+
+### sbt
+
+In `project/plugins.sbt`:
+
+```scala
+addSbtPlugin("org.polyvariant" % "smithy-trait-codegen-sbt" % version)
+```
+
+In `build.sbt`:
+
+```scala
+// on a Java-only module, e.g. autoScalaLibrary := false, crossPaths := false
+.enablePlugins(SmithyTraitCodegenPlugin)
+.settings(
+  smithyTraitCodegenDependencies := Nil,
+  smithyTraitCodegenNamespace := "my.ns",
+  smithyTraitCodegenJavaPackage := "my.pkg",
+)
+```
+
+### mill
+
+Currently not supported in this plugin. See https://github.com/simple-scala-tooling/smithy-trait-codegen-mill/ for an alternative.
+
+## Useful patterns
+
+### Mixing generated and handwritten providers
+
+In case you want to keep some handwritten trait providers, e.g. if you have union traits (which are currently not supported in smithy-trait-codegen), or want to keep binary compatibility on existing traits.
+
+Instead of `META-INF/services/software.amazon.smithy.model.traits.TraitService`, add the providers to
+
+`META-INF/services-input/software.amazon.smithy.model.traits.TraitService`
+
+and add this snippet to include those in the actual, generated file under `META-INF/services`:
+
+```scala
+smithyTraitCodegenExternalProviders ++=
+  IO.readLines(
+    (ThisBuild / baseDirectory).value / "modules" / "protocol" / "resources" / "META-INF" / "services-input" / classOf[TraitService].getName()
+  ).filterNot(_.trim.startsWith("#"))
+    .filterNot(_.trim.isEmpty),
+Compile / packageBin / mappings ~= {
+  _.filterNot { case (_, path) => path.contains("services-input") }
+}
+```
+
+You can also hardcode `smithyTraitCodegenExternalProviders` right there:
+
+```scala
+smithyTraitCodegenExternalProviders ++= List("my.pkg.Trait1$Provider", "my.pkg.Trait2$Provider")
+```