Update CONTRIBUTING.md (#3785) (#3787)

(cherry picked from commit d8991c5)

Co-authored-by: Megan Wachs <megan@sifive.com>

CONTRIBUTING.md

@@ -12,7 +12,7 @@
 
 5. You can peruse the [good-first-issues](https://github.com/chipsalliance/chisel3/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22) for easy tasks to start with. Another easy thing to start with is doing your own pass of the [website](https://www.chisel-lang.org/chisel3/docs/introduction.html) looking for typos, pages missing their titles, etc. The sources for the website are [here](https://github.com/chipsalliance/chisel3/tree/master/docs).
 
-6. Please make your PRs against the `master` branch. The project admins, when reviewing your PR, will decide which stable version (if any) your change should be backported to. The backports will be opened automatically on your behalf once your `master` PR is merged.
+6. Please make your PRs against the `main` branch. The project admins, when reviewing your PR, will decide which stable version (if any) your change should be backported to. They will apply the appropriate `milestone` marker which controls which branches the backport will be opened to. The backports will be opened automatically on your behalf once your `main` PR is merged.
 
 7. The PR template will require you to select "Type of Improvement." A reviewer or someone with write access will add the appropriate label to your PR based on this type of improvement which will include your PR in the correct category in the release notes.