Create init.md

Docs/init.md

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+# Setup instructions
+
+## Step 1. Install CodegenKit into your project
+
+Add `CodegenKit` as dependency of your project.
+
+```swift
+let package = Package(
+    ...
+    dependencies: [
+        .package(url: "https://github.com/omochi/CodegenKit", from: "1.2.0")
+    ]
+    ...
+)
+```
+
+## Step 2. Setup your project
+
+Perform command below.
+
+```
+$ swift package codegen-kit init
+```
+
+This is all that is needed to complete the required setup.
+
+Instead of command, you can do it manually.
+In that case, see instructions described later in this document.
+
+## Step 3. Write placeholder into code
+
+Define placeholders with `@codegen` and `@end` as follows.
+
+```swift
+// TSDecl.swift
+protocol TSDecl {}
+
+extension TSDecl {
+    // @codegen(as)
+    // @end
+}
+```
+
+Attach name of placeholder at `@codegen`.
+
+## Step 4. Implement code renderer
+
+The init command created executable target named `codegen`.
+Add your renderer code into this.
+
+Implement renderers to conform `CodegenKit.Renderer` as follows.
+
+```swift
+import Foundation
+import CodegenKit
+
+struct Node {
+    var stem: String
+    var typeName: String
+}
+
+struct TSDeclRenderer: Renderer {
+    var nodes: [Node] = [...]
+
+    func isTarget(file: URL) -> Bool {
+        file.lastPathComponent == "TSDecl.swift"
+    }
+
+    func render(template: inout CodeTemplate, file: URL, on runner: CodegenRunner) throws {
+        template["as"] = asCasts()
+    }
+
+    func asCasts() -> String {
+        let lines: [String] = nodes.map { (node) in
+            """
+public var as\(node.stem.pascal): \(node.typeName)? { self as? \(node.typeName) }
+"""
+        }
+        return lines.joined(separator: "\n")
+    }
+}
+```
+
+Specify target source code for this renderer with `isTarget` method.
+
+Write rendering logics into `render` method.
+Your source code are passwd as `CodeTemplate` object,
+you can edit content of placeholders via subscript.
+
+CodegenKit automatically format generated code by [swift-format](https://github.com/apple/swift-format).
+So you don't have to worry about precise textual control like indenting when write renderer.
+
+## Step 5. Register your renderers to the runner.
+
+The init command created `main.swift` in `codegen` target.
+The codegen runner is defined in here.
+
+Edit this source to register your renderer to the runner.
+
+```swift
+import CodegenKit
+
+let runner = CodegenRunner(renderers: [
+    TSDeclRenderer()
+])
+let dir = URL(fileURLWithPath: CommandLine.arguments[1])
+try runner.run(directories: [dir])
+```
+
+## Step 6. Perform code generation
+
+The init command created `codegen` command plugin.
+So you can perform code generation as below.
+
+```
+$ swift package codegen
+```
+
+# More advanced code generation
+
+CodegenKit initially creates `codegen` executable and plugin by the init command,
+but has no requirements for these specifications after that.
+
+So, you can modify these targets source and build more complex code generations.
+
+# Setup manually
+
+To setup it manually without using the init command, do the following steps instead.
+
+## Step 1. Create codegen executable
+
+Create `codegen` executable target.
+
+```swift
+let package = Package(
+    ...
+    targets: [
+        .executableTarget(
+            name: "codegen",
+            dependencies: [
+                .product(name: "CodegenKit", package: "CodegenKit")
+            ]
+        ),
+    ...
+)
+```
+
+You can use any target name other than `codegen`.
+
+Write main file that build `CodegenKit.CodegenRunner` and run it.
+
+```swift
+import CodegenKit
+
+let runner = CodegenRunner(renderers: [
+    TSDeclRenderer()
+])
+let dir = URL(fileURLWithPath: CommandLine.arguments[1])
+try runner.run(directories: [dir])
+```
+
+There are no specification about anything including command line arguments.
+Above example build as run on current working directory.
+
+After that, you can generate code as below.
+
+```
+$ swift run codegen .
+```
+
+## Step 2 (Optional). Create command plugin
+
+You can create [command plugin](https://github.com/apple/swift-evolution/blob/main/proposals/0332-swiftpm-command-plugins.md) 
+that perform previous `codegen` executable.
+
+After that, you can generate code as below.
+
+```
+$ swift package codegen
+```
+
+