Update CONTRIBUTING.md

Summary:
Update the contributing doc to reference codereview.cc.

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Closes #52
GitHub author: jverkoey

!![[#52 | This is a child of GitHub pull request #52]]!!.

Reviewers: #material_components_ios_owners, featherless

Reviewed By: #material_components_ios_owners, featherless

Projects: #material_components_ios_owners

Differential Revision: http://codereview.cc/D68

Author: Jeff Verkoeyen

CONTRIBUTING.md

@@ -19,7 +19,7 @@ frustration later on.
 ### Code reviews
 
 All submissions, including submissions by project members, require review. We
-use GitHub pull requests for this purpose.
+allow pull requests to be filed, but we perform code reviews on codereview.cc.
 
 ### The small print