Add reminder to create .abi-compliance-history.

This commit adds a note to RELEASE_CHANGES to remind us to create
an .abi-compliance-history file for new major versions.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: David E. Wheeler <david@justatheory.com>
Discussion: https://postgr.es/m/aPJ03E2itovDBcKX%40nathan

src/tools/RELEASE_CHANGES

@@ -61,6 +61,11 @@ in both master and the branch.
 * Ports
 	o update ports list in doc/src/sgml/installation.sgml
 
+* Create .abi-compliance-history file with initial entry shortly before the .0
+  stamp.  The easiest way to do this is to copy it from the previous
+  REL_*_STABLE branch, remove all entries, and follow the instructions in the
+  file to add the initial reference point for the major version.
+
 
 Pre-Beta Tasks
 ==============