Add gitlab releases page to release checklist.

These make for a nicer landing page for people coming from
the gitlab repo interface on the web. They parallel github
in that we can't override the git snapshots labelled as
'source' but we can link to packages.

Author: rillian

doc/release.txt

@@ -25,6 +25,10 @@ Releases are commited to https://svn.xiph.org/releases/opus/
 which propagates to downloads.xiph.org, and copied manually
 to https://archive.mozilla.org/pub/opus/
 
+Release notes and package links should be added to the corresponding
+tag at https://gitlab.xiph.org/xiph/opusfile so they show on the
+releases page.
+
 Release packages should also be manually attached to the corresponding
 tag on the github mirror https://github.com/xiph/opusfile/releases