README.md: add deprecation notice (#421)

Let's add an explicit deprecation notice so that users of TSC are warned and can start looking for alternatives.

Author: MaxDesiatov

README.md

@@ -4,6 +4,13 @@ swift-tools-support-core
 Contains common infrastructural code for both [SwiftPM](https://github.com/apple/swift-package-manager)
 and [llbuild](https://github.com/apple/swift-llbuild).
 
+## ⚠️ This package is deprecated
+
+As this package with time has become a collection of unrelated utilities, that made it much harder to version.
+Primary users of TSC such as SwiftPM and Swift Driver came up with specialized alternatives to APIs provided
+in TSC. Moving forward, we don't recommend adding TSC as a dependency to your project. More and more types
+and functions here will be deprecated, with minimal modifications to ease the migration off TSC.
+
 License
 -------