Simplified contribution guidelines

CONTRIBUTING.md

@@ -57,17 +57,29 @@ Creating a PR on Huggingface is a bit different from creating one on Github.
 1) Go to the community tab on huggingface press *new pull request* and choose *on your machine*. Specify the title of the your PR. Then you can simply:
 
 ```bash
-git fetch origin refs/pr/{PR NUMBER}:pr/{PR NUMBER}
-git checkout pr/{PR NUMBER}
+git checkout -b {new branch name}
 # make your changes here 
 
 # push to hub
 # you might need to first login:
 # huggingface-cli login
-git push origin pr/{PR NUMBER}:refs/pr/{PR NUMBER}
+git push origin HEAD:refs/pr/{PR NUMBER}
 ```
+Where HEAD refers to the current branch. 
+
+Before you make the PR do be sure to make sure that you have completed the checklist below.
 
-Before you make the PR do be sure to make sure that you have completed the following checklist.
+### Making changes to an existing PR
+
+As a contributor you might need to develop on an existing branch. To do so you you 
+```bash
+# fetch and checkout existing branch:
+git fetch origin refs/pr/{PR NUMBER}:pr/{PR NUMBER}
+git checkout pr/{PR NUMBER}
+# make your changes here
+
+# push changes
+```
 
 ### Checklist