copilot: Update contribution guidelines. Refs Copilot-Language#476.

Copilot now has a process that allows us to accept contributions from
external users again.

This commit updates the README to describe the new process that users
should follow to contribute to Copilot. I am choosing to leave some
details out to not overwhelm users in the README; that can be discussed
later when the PR is processed.

copilot/README.md

@@ -281,12 +281,31 @@ tutorial, please visit the Copilot website:
 # Contributions
 <sup>[(Back to top)](#table-of-contents)</sup>
 
-Copilot cannot accept pull requests or code contributions from developers
-outside the development team at this point.
-
-If you have a question, find a bug, or would like to request a change, please
-file an issue adding as much information as you can to help us reproduce the
-error or identify the use case. Please file the issue with no labels.
+We'd love to receive your contributions, be it code fixes, new features, bug
+reports, discussions, or anything else that you think can help the Copilot
+project.
+
+If you have any comments, questions, ideas, or other topics that you think may
+be of interest, start a new discussion
+[here](https://github.com/Copilot-Language/copilot/discussions).
+
+If you would like to contribute a fix for an issue, please comment on the issue
+indicating that you want to fix it so that we can track it and assign it to
+you. If the issue does not exist, create it first or ask that an existing
+discussion be promoted to an issue.
+
+If you are unsure about whether your submission should be filed as an issue or
+as a discussion, file it as a discussion. We can always move it later.
+
+To facilitate merging any pull requests that you send, please:
+- Reference the issue you are addressing with the text `Refs #<ISSUENUMBER>.`
+  at the end of the subject line of each commit message, in *every commit*
+  (`<ISSUENUMBER>` is the specific issue your pull request is addressing).
+- Describe in the commit messages what you are doing *in each commit*.
+- Update the CHANGELOGs in the *last commit(s)*.
+
+You can take a look at the repository's commit history to better understand the
+process we follow.
 
 # Acknowledgements
 <sup>[(Back to top)](#table-of-contents)</sup>