[Release notes] Mention readability-container-size-empty improvements

[alexfh]

LG with a nit.

docs/ReleaseNotes.rst

94

The wording is a bit too broad. We should somehow hint that the methods are not just arbitrary. Maybe "with suitable `size() and empty()` methods"?

[Eugene.Zelenko]

docs/ReleaseNotes.rst

94

Will fix on commit. Suitable sounds good enough since link to documentation is provided, where more details could be found.

docs/ReleaseNotes.rst

   Warns about the performance overhead arising from concatenating strings using
   the ``operator+``, instead of ``operator+=``.
 
+- `readability-container-size-empty
+  <http://clang.llvm.org/extra/clang-tidy/checks/readability-container-size-empty.html>`_ check
+  supports arbitrary containers with ``empty()`` and ``size()`` methods.
+
 - New `readability-misplaced-array-index
   <http://clang.llvm.org/extra/clang-tidy/checks/readability-misplaced-array-index.html>`_ check