Doc: add pre-branch task to run src/tools/copyright.pl.

It's common for some files with last year's copyright date
to sneak into the tree between early January (when we normally run
copyright.pl) and feature freeze.  Immediately before branching
the new release is an ideal time to fix the stragglers, so add a
note about it to the RELEASE_CHANGES checklist.

Discussion: https://postgr.es/m/CALa6HA4_Wu7-2PV0xv-Q84cT8eG7rTx6bdjUV0Pc=McAwkNMfQ@mail.gmail.com

src/tools/RELEASE_CHANGES

@@ -89,6 +89,9 @@ Starting a New Development Cycle
 * Typically, we do pgindent and perltidy runs just before branching,
   as well as before beta (complete steps from src/tools/pgindent/README)
 
+* It's also advisable to check that copyright years are up-to-date
+  (run src/tools/copyright.pl, commit any changes it finds)
+
 * Create a branch in git for maintenance of the previous release
 	o on master branch, do:
 		git pull           # be sure you have the latest "master"