add info about 'making pull requests'

CONTRIBUTING.md

@@ -15,11 +15,31 @@ community.plot.ly (tagged [`plotly-js`](http://community.plot.ly/c/plotly-js)) o
 [`plotly`](https://stackoverflow.com/questions/tagged/plotly)).
 
 Comments on GitHub issues or pull requests should add content to the discussions. 
-Approbation comments such as *+1* or *I would like this feature to be implemented as well* will be deleted by the maintainers. Please use [GitHub reactions](https://github.com/blog/2119-add-reactions-to-pull-requests-issues-and-comments) instead.
+Approbation comments such as *+1* or *I would like this feature to be implemented as well* 
+will be deleted by the maintainers. Please use 
+[GitHub reactions](https://github.com/blog/2119-add-reactions-to-pull-requests-issues-and-comments) 
+instead.
 
-## Issue labels
+## Making pull requests
 
-We use the following [labels](https://github.com/plotly/plotly.js/labels) to track issues:
+Developers are strongly encouraged to first make a PR to their own plotly.js
+fork and ask one of the maintainers to review the modifications there. Once the
+pull request is deemed satisfactory, the developer will be asked to make a pull
+request to the main plotly.js repo and may be asked to squash some commits
+before doing so.
+
+Developers should `git rebase` their local branch off the latest `master` before 
+opening a pull request.
+
+Note that it is forbidden to force push (i.e. `git push -f`) to remote branches
+associated with opened pull requests. Force pushes make it hard for maintainers 
+to keep track of updates. Therefore, if required, please
+`git merge master` into your PR branch instead of `git rebase master`.
+
+
+## GitHub labels
+
+We use the following [labels](https://github.com/plotly/plotly.js/labels) to track issues and PRs:
 
 | Label | Purpose |
 |--------|---------|