Release process: add another source of info

Normally, the list of CVEs has been reported on our issue already.

wiki/src/contribute/release_process.mdwn

@@ -1470,6 +1470,7 @@ order to let the users of the old versions know that they have to upgrade:
    - if we are not shipping Linux from Debian stable, the list of
      CVE fixed in Linux since the one shipped in the previous release of
      Tails; you can find them in the relevant changelog e.g.:
+     * the GitLab issue that lead to the kernel upgrade
      * <https://metadata.ftp-master.debian.org/changelogs/main/l/linux/unstable_changelog>
      * <https://metadata.ftp-master.debian.org/changelogs/main/l/linux/testing_changelog>
      * <https://metadata.ftp-master.debian.org/changelogs/main/l/linux/buster-backports_changelog>