Add dedicated README for SwiftSyntaxBuilder

Author: akkyie

Sources/SwiftSyntaxBuilder/README.md

@@ -0,0 +1,30 @@
+# SwiftSyntaxBuilder
+
+Declarative and type-safe wrapper around SwiftSyntax.
+
+## Example Usage
+
+```swift
+let sourceFile = SourceFile {
+  Import("SwiftSyntax")
+
+  Struct("ExampleStruct") {
+    Let("syntax", of: "Syntax")
+  }
+}
+
+let syntax = sourceFile.buildSyntax(format: format, leadingTrivia: .zero)
+
+var text = ""
+syntax.write(to: &text)
+print(text)
+```
+
+prints:
+
+```swift
+import SwiftSyntax
+struct ExampleStruct {
+  let syntax: Syntax
+}
+```