Add link to developer docs in readme/contributing.md (#9248)

* Update README.md

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Code Contribution Guidelines
 
-Thank you for your interest in contributing! 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 and [join our "Contributors" community channel](https://pre-release.mattermost.com/core/channels/tickets) to ask questions from community members and the Mattermost core team.
+Thank you for your interest in contributing! Please see the [Mattermost Contribution Guide](https://developers.mattermost.com/contribute/getting-started/) which describes the process for making code contributions across Mattermost projects and [join our "Contributors" community channel](https://pre-release.mattermost.com/core/channels/tickets) to ask questions from community members and the Mattermost core team.
 
 ### Review Process for this Repo