Create CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+We welcome and appreciate input, ideas, and code from contributors.
+
+For any user-facing change, please [submit
+an issue for discussion first](https://github.com/mozilla/fx-private-relay/issues/new). To keep our review
+queue streamlined, if you submit your idea as a pull request with code changes,
+we are likely to close it and ask you to submit an issue for discussion first.
+
+Before you submit a new issue, please 
+[search our existing issues](https://github.com/mozilla/fx-private-relay/issues)
+to see if your idea is already in there. If it is, please add a +1 vote to the
+existing issue. We check our [issues sorted by
+upvotes](https://github.com/mozilla/fx-private-relay/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc)
+when we plan and prioritize our work.
+
+If you submit a PR for code changes, we will try to respond as quickly as we
+can, but please be patient - especially if your change is large.
+
+## Getting started with the code
+Our instructions for installing, running, and testing code are in
+[README.md](https://github.com/mozilla/fx-private-relay/blob/main/README.md).https://github.com/mozilla/fx-private-relay/blob/main/README.md