ReleaseNotes: Document removal of add_llvm_loadable_module CMake macro

This was removed in r349839. llvm-svn: 349921

Author: tstellar

llvm/docs/ReleaseNotes.rst

@@ -43,6 +43,11 @@ Non-comprehensive list of changes in this release
 * The **llvm-cov** tool can now export lcov trace files using the
  `-format=lcov` option of the `export` command.
 
+* The add_llvm_loadable_module CMake macro has been removed. The
+ add_llvm_library macro with the MODULE argument now provides the same
+ functionality. See `Writing an LLVM Pass
+ <WritingAnLLVMPass.html#setting-up-the-build-environment>`_.
+
 .. NOTE
  If you would like to document a larger change, then you can add a
  subsection about it right here. You can copy the following boilerplate