Update readme describing how generate-swiftsyntax with HEAD works

Author: kimdv

CodeGeneration/README.md

@@ -4,7 +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.
 
-This directory is a standalone package that uses a pinned version of SwiftSyntaxBuilder. It is thus NOT using SwiftSyntaxBuilder of the parent directory. This guarantees that when `generate-swiftsyntax` is run, it can't break its own build.
+This directory is a standalone package that uses HEAD of the current branch. This guarantees that when `generate-swiftsyntax` is run, it can't break its own build when run multiple times without committing. 
+This means that `CodeGeneration` will build against your latest local commit of SwiftSyntax. If you are making changes to `SwiftSyntax` that affect how code is being generated, commit your SwiftSyntax changes (pushing is not necessary) and re-generate files afterwards.
 
 To re-generate the files after changing `CodeGeneration` run the `generate-swiftsyntax` 
 target of `CodeGeneration` and pass `path/to/swift-syntax/Sources` as the argument.