CONTRIBUTING.md: add Forking definition

CONTRIBUTING.md

@@ -3,6 +3,12 @@
 When contributing to this repository, please first discuss the change you wish to make via issues,
 email, or on the [ENS-Bioinfo channel](https://matrix.to/#/#ens-bioinfo:matrix.org) before making a change. 
 
+## Forking
+
+In git, the [action of forking](https://git-scm.com/book/en/v2/GitHub-Contributing-to-a-Project) means that you are going to make your own private copy of a repository. You can then write modifications in your project, and if they are of interest for the source repository create a merge request (here [LBMC/nextflow](https://gitbio.ens-lyon.fr/LBMC/nextflow)). Merge requests are sent to the source repository to ask the maintainers to integrate modifications.
+
+![merge request button](img/merge_request.png)
+
 ## Project organization
 
 The `LBMC/nextflow` project is structured as follows: