Update contributing.md (#4203)

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

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 # Code Contribution Guidelines
 
 Please see the [Mattermost Contribution Guide](http://docs.mattermost.com/developer/contribution-guide.html) which describes the process for making code contributions across Mattermost projects. 
+
+### Review Process for this Repo
+
+After following the steps in the [Contribution Guide](http://docs.mattermost.com/developer/contribution-guide.html), submitted pull requests go through the review process outlined below. We aim to start reviewing pull requests in this repo the week they are submitted, but the length of time to complete the process will vary depending on the pull request.
+
+The one exception may be around release time, where the review process may take longer as the team focuses on our [release process](https://docs.mattermost.com/process/release-process.html). 
+
+#### `Stage 1: PM Review`
+
+A Product Manager will review the pull request to make sure it:
+
+1. Fits with our product roadmap
+2. Works as expected
+3. Meets UX guidelines
+
+This step is sometimes skipped for bugs or small improvements with a ticket, but always happens for new features or pull requests without a related ticket. 
+
+The Product Manager may come back with some bugs or UI improvements to fix before the pull request moves on to the next stage.
+
+#### `Stage 2: Dev Review`
+
+Two developers will review the pull request and either give feedback or `+1` the PR.
+
+Any comments will need to be addressed before the pull request moves on to the last stage.
+
+#### `Stage 3: Ready to Merge`
+
+The review process is complete, and the pull request will be merged.