doc/developer: Add "security fixes" to the release process steps

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/developer.texi

@@ -642,6 +642,9 @@ The release process involves the following steps:
     Add the release at @url{https://ffmpeg.org/trac/ffmpeg/admin/ticket/versions}.
 @item
     Announce the intent to do a release to the mailing list.
+@item
+    Make sure all relevant security fixes have been backported. See
+    @url{https://ffmpeg.org/security.html}.
 @item
     Ensure that the FATE regression suite still passes in the release
     branch on at least @strong{i386} and @strong{amd64}