Improve gem update explanation

CHANGELOG.md

@@ -7,7 +7,9 @@ features.
 ## 2016-10-31
 - Add root check to catch root move problems. Requires gem 0.2.3 or
-  newer; run `gem install gitlab-development-kit` to upgrade the gem.
+  newer. Next time you run `git pull` in the gitlab-development-kit
+  directory, also run `gem install gitlab-development-kit` to upgrade
+  the gem.
 ## 2016-09-09
 - Update Procfile for gitlab_workhorse_secret

[Jacob Vosmaer (GitLab)]

Maybe we should say 'root directory' here?

[Zeger-Jan van de Weg]

I think the current wording is best.

[Luke "Jared" Bennett]

I like gitlab-development-kit > root.

[Jacob Vosmaer (GitLab)]

Sorry, what I meant was: 'in the gitlab-development-kit root directory'.