Add a document with contribution guidelines

CONTRIBUTING.md

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+# Contributing to nix
+
+We're really glad you're interested in contributing to nix! This
+document has a few pointers and guidelines to help get you started.
+
+To have a welcoming and inclusive project, nix uses the Rust project's
+[Code of Conduct][conduct]. All contributors are expected to follow it.
+
+[conduct]: https://www.rust-lang.org/conduct.html
+
+
+# Issues
+
+We use GitHub's [issue tracker][issues]. 
+
+[issues]: https://github.com/nix-rust/nix/issues
+
+
+## Bug reports
+
+Before submitting a new bug report, please [search existing
+issues][issue-search] to see if there's something related. If not, just
+[open a new issue][new-issue]!
+
+As a reminder, the more information you can give in your issue, the
+easier it is to figure out how to fix it. For nix, this will likely
+include the OS and version, and the architecture.
+
+[issue-search]: https://github.com/nix-rust/nix/search?utf8=%E2%9C%93&q=is%3Aissue&type=Issues
+[new-issue]: https://github.com/nix-rust/nix/issues/new
+
+
+## Feature / API requests
+
+If you'd like a new API or feature added, please [open a new
+issue][new-issue] requesting it. As with reporting a bug, the more
+information you can provide, the better.
+
+
+## Labels
+
+We use labels to help manage issues. The structure is modeled after
+[Rust's issue labeling scheme][rust-labels]:
+- **A-**prefixed labels state which area of the project the issue
+  relates to
+- **O-**prefixed labels specify the OS for issues that are OS-specific
+
+[rust-labels]: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#issue-triage
+
+
+# Pull requests
+
+We use GitHub's pull requests for making changes to nix.
+
+All pull requests are merged via [homu], an integration bot. After the
+pull request has been reviewed, the reviewer will leave a comment like
+
+> @homu r+
+
+to let @homu know that it was approved. Then @homu will check that it
+passes tests, and merge if the tests succeed.
+
+[homu]: https://github.com/nix-rust/nix/blob/master/CONVENTIONS.md
+
+
+## API conventions
+
+If you're adding a new API, we have a [document with
+conventions][conventions] to use throughout the nix project.
+
+[conventions]: https://github.com/nix-rust/nix/blob/master/CONVENTIONS.md