pgsql: Mention use of src/tools/major_release_split for creating

Log Message:
-----------
Mention use of src/tools/major_release_split for creating back-branch
release notes.

Modified Files:
--------------
pgsql/src/tools:
RELEASE_CHANGES (r1.80 -> r1.81)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/RELEASE_CHANGES?r1=1.80&r2=1.81)