Add link to developer docs in readme/contributing.md (#1560)

* 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:https://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: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
 

README.md

@@ -7,6 +7,7 @@ It's written in Golang and React and runs as a single Linux binary with MySQL or
 This project hosts the webapp client code. Please file issues at [/mattermost-server](https://github.com/mattermost/mattermost-server), which hosts the server code.
 
 - [Review product documentation](http:https://docs.mattermost.com/).
+- [Review developer documentation](https://developers.mattermost.com/).
 - [Download compiled version](https://mattermost.org/download).
 
 Try out Mattermost: