Add contributing guidelines reference to readme (#753)

* Add contributing guidelines reference to readme

* Tweak

Author: raunofreiberg

README.md

@@ -24,6 +24,10 @@ This repo contains the source code and documentation powering [reactjs.org](http
 
 ## Contributing
 
+### Guidelines
+
+The documentation is divided into several sections with a different tone and purpose. If you plan to write more than a few sentences, you might find it helpful to get familiar with the [contributing guidelines](https://github.com/reactjs/reactjs.org/blob/master/CONTRIBUTING.md#guidelines-for-text) for the appropriate sections.
+
 ### Create a branch
 
 1. `git checkout master` from any folder in your local `reactjs.org` repository