Update README.md (#16224)

addition of contributing section

README.md

@@ -22,6 +22,7 @@ Table of Contents
   * [Get Involved](#get-involved)
   * [Learn More](#learn-more)
   * [Get the Latest News](#get-the-latest-news)
+  * [Contributing](#contributing)
 
 ## Try out Mattermost
 
@@ -85,4 +86,7 @@ See the [LICENSE file](LICENSE.txt) for license rights and limitations.
 - **Mattermost** - Join the ~contributors channel on [the Mattermost Community Server](https://community.mattermost.com). 
 - **IRC** - Join the #matterbridge channel on [Freenode](https://freenode.net/) (thanks to [matterircd](https://github.com/42wim/matterircd)).
 
+## Contributing
+Please see [CONTRIBUTING.md](./CONTRIBUTING.md).
+
 Any other questions, mail us at info@mattermost.com. We’d love to meet you!