Move footnote from CONTRIBUTING.md to doc/development/code_review.md

A section was moved in f644b8c8 but this got lost.

CONTRIBUTING.md

@@ -728,6 +728,3 @@ available at [http://contributor-covenant.org/version/1/1/0/](http://contributor
 [polling-etag]: https://docs.gitlab.com/ce/development/polling.html
 [testing]: doc/development/testing_guide/index.md
 [us-english]: https://en.wikipedia.org/wiki/American_English
-
-[^1]: Please note that specs other than JavaScript specs are considered backend
-      code.

doc/development/code_review.md

@@ -207,3 +207,4 @@ Largely based on the [thoughtbot code review guide].
 [projects]: https://about.gitlab.com/handbook/engineering/projects/
 [team]: https://about.gitlab.com/team/
 [build handbook]: https://about.gitlab.com/handbook/build/handbook/build#how-to-work-with-build
+[^1]: Please note that specs other than JavaScript specs are considered backend code.