Updated CONTRIBUTING.md with fork/PR submission info

To make it clear on how others can contribute to the project there's
now a dedicated section on how to raise a PR including a link to GitHub's
documentation on forking repos

Author: adammcarter

CONTRIBUTING.md

@@ -9,3 +9,13 @@ Before submitting the pull request, please make sure you have [tested your
 changes](https://github.com/apple/swift/blob/main/docs/ContinuousIntegration.md)
 and that they follow the Swift project [guidelines for contributing
 code](https://swift.org/contributing/#contributing-code).
+
+---
+
+## Opening a Pull Request
+
+To submit a Pull Request (PR) you don't need permissions on this repo, instead you can fork the repo and create a PR through your forked version.
+
+For more information and instructions, read the GitHub docs on [forking a repo](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo).
+
+Once you've pushed your branch, you should see an option on this repository's page to create a PR from a branch in your fork.