[libclang/python] Add release notes

Author: DeinAlptraum

clang/docs/ReleaseNotes.rst

@@ -54,6 +54,11 @@ Clang Frontend Potentially Breaking Changes
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- Parts of the interface returning string results will now return
+  the empty string `""` when no result is available, instead of `None`.
+- Calling a property on the `CompletionChunk` or `CompletionString` class
+  statically now leads to an error, instead of returning a `CachedProperty` object
+  that is used internally. Properties are only available on instances.
 
 What's New in Clang |release|?
 ==============================