Added release notes.

Author: thomashampson

docs/source/release/v6.14.0/Framework/Data_Objects/New_features/39488.rst

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+- Workspace names that do not conform to python variable naming conventions
+  (must start with letter or underscore, and contain only letters, digits and underscores)
+  will invoke a warning message.
+  In a future version of mantid, invalid workspace names will cause an exception to be raised.
+  Please update your scripts to use valid Python identifiers for workspace names.