add note about git rebase before making a PR

Author: etpinard

CONTRIBUTING.md

@@ -28,15 +28,18 @@ 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.
 
-It is forbidden to force push (i.e. `git push -f`) to remote branches
+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`.
+to keep track of updates. Therefore, if required, please
+`git merge master` into your PR branch instead of `git rebase master`.
 
 
-## Issue labels
+## GitHub labels
 
-We use the following [labels](https://github.com/plotly/plotly.js/labels) to track issues:
+We use the following [labels](https://github.com/plotly/plotly.js/labels) to track issues and PRs:
 
 | Label | Purpose |
 |--------|---------|