Add README.md for trait codegen

Author: joewyz

smithy-trait-codegen/README.md

@@ -0,0 +1,141 @@
+# Smithy Trait CodeGen
+
+Smithy build plugin that generates Java trait classes from customized Smithy trait defined with [`@trait`](https://smithy.io/2.0/spec/model.html#trait-trait).
+
+## Supported Trait Types
+- [**Simple Types**](https://smithy.io/2.0/spec/simple-types.html) (Excluding `Blob` and `Boolean`)
+- **Structure**
+- **List**
+- **Set**
+- **Map**
+
+> [!NOTE]
+> [Annotation Trait](https://smithy.io/2.0/spec/model.html#annotation-traits) is recommended rather than Boolean trait
+
+## Supported Smithy Prelude Traits
+- [@required](https://smithy.io/2.0/spec/type-refinement-traits.html#required-trait)
+- [@enumValue](https://smithy.io/2.0/spec/type-refinement-traits.html#enumvalue-trait)
+- [@mixin](https://smithy.io/2.0/spec/type-refinement-traits.html#mixin-trait)
+- [@idRef](https://smithy.io/2.0/spec/constraint-traits.html#idref-trait)
+- [@uniqueItems](https://smithy.io/2.0/spec/constraint-traits.html#uniqueitems-trait)
+- [@deprecatedTrait](https://smithy.io/2.0/spec/documentation-traits.html#deprecated-trait)
+- [@documentation](https://smithy.io/2.0/spec/documentation-traits.html#documentation-trait)
+- [@externalDocumentation](https://smithy.io/2.0/spec/documentation-traits.html#externaldocumentation-trait)
+- [@timestampFormat](https://smithy.io/2.0/spec/protocol-traits.html#timestampformat-trait)
+
+## Configuration
+
+The generator supports the following configuration options: 
+
+| Option | Description | Required |
+|--------|-------------|----------|
+| `package` | Java package name for generated classes | Yes |
+| `namespace` | Smithy namespace to search for traits | Yes |
+| `header` | Header lines to include in generated files | No |
+| `excludeTags` | Smithy tags to exclude from generation | No |
+
+An example for `smithy-build.json`:
+
+```json
+{
+    "version": "1.0",
+    "plugins": {
+        "trait-codegen": {
+            "package": "com.example.traits",
+            "namespace": "com.example.traits",
+            "header": ["Copyright Example Corp"],
+            "excludeTags": ["internal"]
+        }
+    }
+}
+```
+
+## Getting Started
+
+First, create a gradle-based Smithy model project. You can also use Smithy CLI to generate a [complete custom trait example](https://github.com/smithy-lang/smithy-examples/tree/main/custom-trait-examples/custom-trait): `smithy init -o custom-trait -t custom-trait`.
+
+> [!NOTE]
+> The generator currently cannot be run with non-gradle-based projects.
+
+In `build.gradle.kts`, add `id("software.amazon.smithy.gradle.smithy-trait-package").version("1.3.0")` to the `plugin` section.
+```kotlin
+plugins {
+    id("software.amazon.smithy.gradle.smithy-trait-package").version("1.3.0")
+}
+
+dependencies {
+    smithyCli("software.amazon.smithy:smithy-cli:1.61.0")
+    implementation("software.amazon.smithy:smithy-model:1.61.0")
+}
+
+repositories {
+    mavenLocal()
+    mavenCentral()
+}
+```
+
+Next, add the `trait-codegen` plugin to the
+[plugin configuration](https://smithy.io/2.0/guides/smithy-build-json.html) in `smithy-build.json`.
+
+```json
+"plugins": {
+    "trait-codegen": {
+        "package": "io.examples.traits",
+        "namespace": "example.traits",
+    }
+}
+```
+
+Finally, build the model with Gradle: `gradle build` and you can find the the generated Java classes under `/build/smithyprojections/trait-codegen/`
+
+The generated Java class for `annotationTrait` from `custom-trait` template would be:
+```java
+@SmithyGenerated
+public final class AnnotationTrait extends AbstractTrait implements ToSmithyBuilder<AnnotationTrait> {
+    public static final ShapeId ID = ShapeId.from("example.traits#annotationTrait");
+
+    private AnnotationTrait(Builder builder) {
+        super(ID, builder.getSourceLocation());
+    }
+
+    @Override
+    protected Node createNode() {
+        return Node.objectNodeBuilder()
+            .sourceLocation(getSourceLocation())
+            .build();
+    }
+
+    public static AnnotationTrait fromNode(Node node) {
+        Builder builder = builder().sourceLocation(node);
+        node.expectObjectNode();
+        return builder.build();
+    }
+
+    // Builder Implementation
+
+    // Service Provider Implementation
+}
+```
+The generated Java class for `JsonNameTrait` from `custom-trait` template would be:
+```java
+@SmithyGenerated
+public final class JsonNameTrait extends StringTrait {
+    public static final ShapeId ID = ShapeId.from("example.traits#jsonName");
+
+    public JsonNameTrait(String value) {
+        super(ID, value, SourceLocation.NONE);
+    }
+
+    public JsonNameTrait(String value, FromSourceLocation sourceLocation) {
+        super(ID, value, sourceLocation);
+    }
+
+    // Builder Implementation
+
+    // Service Provider Implementation
+}
+```
+
+## License
+
+Licensed under the Apache 2.0 License.