Merge pull request #536 from bashtage/rls-5.1

RLS: Release 5.1

Author: bashtage

doc/source/changes/5.0.rst

@@ -4,8 +4,8 @@
    Variables will appear in results in the same order they are entered in
    the formula string.
 
-Since Version 5.1
------------------
+Version 5.1
+-----------
 * Fixed a bug that affected the counting of the number of entities and time periods
   in panel models (:issue:`534`).