Help: Add 3.1 release note about '#' escaping in Makefiles (#15322)

The change in commit v3.1.0-rc1~174^2 (Makefile: Handle '#' in
COMPILE_OPTIONS, 2014-08-12) was not compatible with code that
tries to workaround the original bug.  Unfortunately there is no
good way to fix the bug in a compatible way.  Add a release note
to call attention to the change.
Merge topic 'doc-octothorpe-escaping' into next

8d70a914 Help: Add 3.1 release note about '#' escaping in Makefiles (#15322)