Fix UX Guide link on Contributing.md

Current link on the CONTRIBUTING.md page was to the old UI Guide. This should be updated to the new link.

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• 4d3ac385 - [ci skip] Fix UX Guide link on Contributing.md

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@axil: I noticed that the link in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md pointed to the old UI guide. I wanted to update this to the new UX guide. Does it make sense to link to the docs.gitlab.com URL (http://docs.gitlab.com/ce/development/ux_guide/) or the /help link? If my changes make sense here, feel free to merge. Thanks!

Reassigned to @axil

Status changed to merged

Mentioned in commit 3255401e

@awhildy yes! docs.gitlab.com :)