Merge pull request swiftlang#2642 from ahoppen/update-readme-for-codegen

Update readme for CodeGeneration

Author: ahoppen

CONTRIBUTING.md

@@ -49,7 +49,7 @@ If you want to modify the generated files, open the [CodeGeneration](CodeGenerat
 Alternatively, you can generate the files from the command line by running the following command from the swift-syntax directory
 
 ```bash
-swift run --package-path CodeGeneration
+./swift-syntax-dev-utils generate-source-code
 ```
 
 ## Running Pre-PR Checks Script

CodeGeneration/README.md

@@ -4,14 +4,8 @@ This directory contains file to generate source code that is part of the SwiftSy
 
 Some source code inside SwiftSyntax is generated using [SwiftSyntaxBuilder](../Sources/SwiftSyntaxBuilder), a Swift library whose purpose is to generate Swift code using Swift itself. This kind of code generation is performed by the Swift package defined in this directory.
 
-To re-generate the files after changing `CodeGeneration` run the `generate-swift-syntax` 
-target of `CodeGeneration` and pass `path/to/swift-syntax/Sources` as the argument.
+To re-generate the files after changing `CodeGeneration` run the following command on the command line.
 
-On the command line, this would be
 ```bash
-swift run --package-path CodeGeneration generate-swift-syntax Sources
+path/to/swift-syntax/swift-syntax-dev-utils generate-source-code
 ```
-
-Or if you open the `CodeGeneration` package in Xcode, you can add the 
-`generate-swift syntax ../Sources` arguments using Product -> Scheme -> Edit Scheme…
-