[ASTGen] Add a comment about the purporse of its Package.swift

Author: DougGregor

lib/ASTGen/Package.swift

@@ -1,5 +1,12 @@
 // swift-tools-version: 5.6
 
+// The CMake build system is the only one that's able to produce a working
+// compiler. This Package.swift makes it easier to build and work with the
+// swiftASTGen library within IDEs, but it's mainly there for editing---it
+// won't create something that can be meaningfully executed. Most things with
+// the new Swift parser are better implemented/tested within or on top of the
+// swift-syntax package.
+
 import PackageDescription
 
 let package = Package(