CMake: document the library name changes as incompatible change

docs/changes.txt

@@ -177,6 +177,13 @@ NOTE: This file is updated only before the release, please use
     $ git notes --ref=changelog add -m 'wxPort: description.' to update it.
 
 
+INCOMPATIBLE CHANGES SINCE 3.1.3:
+
+- CMake library targets were renamed and now start with 'wx'. In addition,
+  aliases for the libraries have been added using the 'wx::' namespace. For
+  example, the core library is now named wxcore and has alias wx::core.
+
+
 3.1.3: (released 2019-10-28)
 ----------------------------