Add a CONTRIBUTING.md file (#479)

* Add a CONTRIBUTING.md file.

* Document the basic PR process.

This also introduces the Core Team.

cranelift/CONTRIBUTING.md

@@ -0,0 +1,117 @@
+# Contributing to Cranelift
+
+## Welcome!
+
+Cranelift is a very ambitious project with many goals, and while we're
+confident we can achieve some of them, we see many opportunities for people
+to get involved and help us achieve even more.
+
+### Ask questions! Give feedback!
+
+This is a relatively young project, and not everything we hope to do with it
+is reflected in the code or documentation yet. If you see things that seem
+missing or that don't make sense, or even that just don't work the way you
+expect them to, we're interested to hear about it!
+
+We have a [Cranelift chat on Gitter], and questions are also welcome as issues
+in the [Cranelift issue tracker]. Some folks also hang out in the #cranelift
+IRC channel on [irc.mozilla.org].
+
+[Cranelift chat on Gitter]: https://gitter.im/CraneStation/Lobby
+[Cranelift issue tracker]: https://github.com/CraneStation/cranelift/issues/new
+[irc.mozilla.org]: https://wiki.mozilla.org/IRC
+
+### Mentoring
+
+We're happy to mentor people, whether you're learning Rust, learning about
+compiler backends, learning about machine code, learning about how Cranelift
+does things, or all together at once.
+
+We tag issues in the issue tracker marked [good first issue] when we can, so
+that's sometimes a good place to get started. Also, we encourage people to just
+look around and find things they're interested in. This a good time to get
+involved, as there aren't a lot of things set in stone yet.
+
+[good first issue]: https://github.com/CraneStation/cranelift/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22
+
+## Coding Guidelines
+
+For the most part, Cranelift follows common Rust conventions and
+[pull request] (PR) workflows, though we do have a few additional things to
+be aware of.
+
+[pull request]: https://help.github.com/articles/about-pull-requests/
+
+### rustfmt
+
+All PRs must be formatted according to the current stable [rustfmt], and this
+is checked in the continuous integration tests. See the [rustfmt quickstart]
+for setup.
+
+[format-all.sh] is a script for running the appropriate version of rustfmt,
+which may be convenient when there are multiple versions installed.
+
+[rustfmt]: https://github.com/rust-lang-nursery/rustfmt
+[rustfmt quickstart]: https://github.com/rust-lang-nursery/rustfmt#quick-start
+[format-all.sh]: https://github.com/CraneStation/cranelift/blob/master/format-all.sh
+
+### Rustc version support
+
+Our current policy is to support the version of Rustc that ships with the
+latest Ubuntu LTS release, as well as the current stable version. This means
+we don't use some of the very latest released Rust features.
+
+Some of the developer scripts depend on nightly Rust, for example to run
+clippy and other tools, however we avoid depending on these for the main
+build.
+
+That said, if there are any new Rust features that would be particularly
+valuable to use, please bring them up, as we may be able to find ways to
+accommodate them.
+
+### Python
+
+Our Python code is checked with [mypy](http://mypy-lang.org/) and
+[flake8](http://flake8.pycqa.org/en/latest/); see the
+[check.sh](https://github.com/CraneStation/cranelift/blob/master/lib/codegen/meta-python/check.sh)
+file for details. The versions available in common package repositories such
+as Ubuntu or Homebrew typically work fine.
+
+## Development Process
+
+We use [issues] for asking questions and tracking bugs and unimplemented
+features, and [pull requests] (PRs) for tracking and reviewing code
+submissions.
+
+When submitting PRs:
+
+ - Write clear commit messages that start with a one-line summary of the
+   change (and if it's difficult to summarize in one line, consider
+   splitting the change into multiple PRs), optionally followed by
+   additional context. Good things to mention include which areas of the
+   code are affected, which features are affected, and anything that
+   reviewers might want to pay special attention to.
+
+ - If there is code which needs explanation, prefer to put the explanation in
+   a comment in the code, or in documentation, rather than in the commit
+   message.
+
+ - For pull requests that fix existing issues, use [issue keywords]. Note that
+   not all pull requests need to have accompanying issues.
+
+Anyone may submit a pull request, and anyone may comment on or review others'
+pull requests. Pull requests are merged by members of the [Core Team].
+
+We generally squash sequences of incremental-development commits together into
+logical commits (though keeping logical commits focused). Developers may do
+this themselves before submitting a PR or during the PR process, or Core Team
+members may do it when merging a PR. Ideally, the continuous-integration tests
+should pass at each logical commit.
+
+Core Team members may push minor changes directly, though should create PRs
+for significant changes.
+
+[issues]: https://guides.github.com/features/issues/
+[pull requests]: https://help.github.com/articles/about-pull-requests/
+[issue keywords]: https://help.github.com/articles/closing-issues-using-keywords/
+[Core Team]: https://github.com/orgs/CraneStation/people/