Add a paragraph on "Implementing design & UI elements" in CONTRIBUTING.md

Also, link to this new paragraph in PROCESS.md and improved a bit this
document.

[ci skip]

CONTRIBUTING.md

@@ -8,6 +8,9 @@
     - [Closing policy for issues and merge requests](#closing-policy-for-issues-and-merge-requests)
     - [Helping others](#helping-others)
     - [I want to contribute!](#i-want-to-contribute)
+    - [Implement design & UI elements](#implement-design-ui-elements)
+        - [Design reference](#design-reference)
+        - [UI development kit](#ui-development-kit)
     - [Issue tracker](#issue-tracker)
         - [Feature proposals](#feature-proposals)
         - [Issue tracker guidelines](#issue-tracker-guidelines)
@@ -83,6 +86,22 @@ GitLab.
 
 This was inspired by [an article by Kent C. Dodds][medium-up-for-grabs].
 
+## Implement design & UI elements
+
+### Design reference
+
+The GitLab design reference can be found in the [gitlab-design] project.
+The designs are made using Antetype (`.atype` files). You can use the
+[free Antetype viewer (Mac OSX only)] or grab an exported PNG from the design
+(the PNG is 1:1).
+
+The current designs can be found in the [`gitlab1.atype` file].
+
+### UI development kit
+
+Implemented UI elements can also be found at https://gitlab.com/help/ui. Please
+note that this page isn't comprehensive at this time.
+
 ## Issue tracker
 
 To get support for your particular problem please use the
@@ -299,13 +318,14 @@ to us than having a minimal commit log. The smaller an MR is the more likely it
 is it will be merged (quickly). After that you can send more MRs to enhance it.
 
 For examples of feedback on merge requests please look at already
-[closed merge requests][closed-merge-requests]. If you would like quick feedback on your merge
-request feel free to mention one of the Merge Marshalls of the [core team][core-team].
+[closed merge requests][closed-merge-requests]. If you would like quick feedback
+on your merge request feel free to mention one of the Merge Marshalls in the
+[core team][core-team] or one of the
+[Merge request coaches](https://about.gitlab.com/team/).
 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 guidelines](https://github.com/thoughtbot/guides/tree/master/code-review)
-into account.
+[Thoughtbot code review guide] into account.
 
 ### Merge request description format
 
@@ -473,3 +493,7 @@ available at [http://contributor-covenant.org/version/1/1/0/](http://contributor
 [rss-source]: https://github.com/bbatsov/ruby-style-guide/blob/master/README.md#source-code-layout
 [rss-naming]: https://github.com/bbatsov/ruby-style-guide/blob/master/README.md#naming
 [doc-styleguide]: doc/development/doc_styleguide.md "Documentation styleguide"
+[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