Add a link to how to manage cog

howto/tweeting-guidelines.md

@@ -4,7 +4,7 @@
 We should always be tweeting from `@gitlabstatus` in an informative but
 reassuring way. We don't tweet directly from Twitter, but via `@marvin`,
-a Cog app on Slack. See [How to tweet](#how-to-tweet) for the commands
+a [Cog][] app on Slack. See [How to tweet](#how-to-tweet) for the commands
 we're using.
 Avoid using ambiguous messages, but if we don't know what is going on yet just state that we are investigating.
@@ -21,6 +21,8 @@ When we have issues with any production operation we should always tweet opening
 Closing tweets should be as detailed as possible, for example: 'we have a failing database migration of a column name'
 instead of 'there is a problem with the database'
+[Cog]: https://gitlab.com/gitlab-com/runbooks/blob/master/howto/manage-cog.md
+
 ## How to tweet
 The one who commands `@marvin` on Slack should get a Cog user. Then we send