Add a contributing guide

CONTRIBUTING.md

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+# Contributing to Serde
+
+Serde welcomes contribution from everyone. Here are the guidelines if you are
+thinking of helping us:
+
+## Contributions
+
+Contributions to Serde or its dependencies should be made in the form of GitHub
+pull requests. Each pull request will be reviewed by a core contributor
+(someone with permission to land patches) and either landed in the main tree or
+given feedback for changes that would be required. All contributions should
+follow this format, even those from core contributors.
+
+Should you wish to work on an issue, please claim it first by commenting on
+the GitHub issue that you want to work on it. This is to prevent duplicated
+efforts from contributors on the same issue.
+
+## Pull Request Checklist
+
+- Branch from the master branch and, if needed, rebase to the current master
+  branch before submitting your pull request. If it doesn't merge cleanly with
+  master you may be asked to rebase your changes.
+
+- Commits should be as small as possible, while ensuring that each commit is
+  correct independently (i.e., each commit should compile and pass tests). 
+
+- If your patch is not getting reviewed or you need a specific person to review
+  it, you can @-reply a reviewer asking for a review in the pull request or a
+  comment, or you can ask for a review in `#serde` on `irc.mozilla.org`.
+
+- Add tests relevant to the fixed bug or new feature.
+
+## Conduct
+
+In all Serde-related forums, we follow the [Rust Code of
+Conduct](https://www.rust-lang.org/conduct.html).  For escalation or moderation
+issues, please contact Erick (erick.tryzelaar@gmail.com) instead of the Rust
+moderation team.
+
+## Communication
+
+Beyond opening tickets on the
+[serde-rs/serde](https://github.com/serde-rs/serde) project, Serde contributors
+frequent the `#serde` channel on
+[`irc.mozilla.org`](https://wiki.mozilla.org/IRC).