doc: document the actual workflow in CONTRIBUTING.md (#468)

Author: bfabio

CONTRIBUTING.md

@@ -26,22 +26,14 @@ If you want to add to the documentation or code of one of our projects you
 should make a Pull Request. 
 
 If you never used GitHub, get up to speed with [Understanding the GitHub
-Flow](https://guides.github.com/introduction/flow/) or follow one of the great
-free interactive courses in the [GitHub learning lab](https://lab.github.com/)
-on working with GitHub and working with MarkDown, the syntax this project's
-documentation is in.
+Flow](https://guides.github.com/introduction/flow/).
 
 This project is [licenced AGPL-3.0-or-later](LICENSE.md), which means you are
 granted the freedoms to read, execute, change and redistributed the code with
 the same license. 
 
 ### 1. Make your changes
 
-This project uses the [**GitFlow branching model** and
-workflow](http://nvie.com/posts/a-successful-git-branching-model/). When you've
-forked this repository, please make sure to create a feature branch following
-the GitFlow model.
-
 Add your changes in commits [with a message that explains
 them](https://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message).
 Document choices or decisions you make in the commit message, this will enable