Link to gitlab code review guide in contributing guidelines

I noticed that the contributing guidelines still link to the thoughtbot
code review guidelines even though we already have our own.

[ci skip]

CONTRIBUTING.md

@@ -349,7 +349,7 @@ on your merge request feel free to mention one of the Merge Marshalls in the
 Please ensure that your merge request meets the contribution acceptance criteria.
 
 When having your code reviewed and when reviewing merge requests please take the
-[Thoughtbot code review guide] into account.
+[code review guidelines](doc/development/code_review.md) into account.
 
 ### Merge request description format
 
@@ -523,4 +523,3 @@ available at [http://contributor-covenant.org/version/1/1/0/](http://contributor
 [gitlab-design]: https://gitlab.com/gitlab-org/gitlab-design
 [free Antetype viewer (Mac OSX only)]: https://itunes.apple.com/us/app/antetype-viewer/id824152298?mt=12
 [`gitlab1.atype` file]: https://gitlab.com/gitlab-org/gitlab-design/tree/master/gitlab1.atype/
-[Thoughtbot code review guide]: https://github.com/thoughtbot/guides/tree/master/code-review