PHP7 sounds like a good time to include signatures in announce mails

We need these posted as many places as possible to make any sense.

Author: bjori

README.RELEASE_PROCESS

@@ -266,7 +266,9 @@ to upgrade.
 
 8. Commit all the changes to their respective git repos
 
-9. Wait an hour or two, then send a mail to php-announce@lists.php.net,
+9. Please note down the md5, sha245 and the PGP signature (.asc). These *must* be
+   included in the release mail.
+10. Wait an hour or two, then send a mail to php-announce@lists.php.net,
 php-general@lists.php.net and internals@lists.php.net with a text similar to
 http://news.php.net/php.internals/17222.
 Please make sure that the mail to php-announce@ is its own completely seperate email.