docs: Document contributing expectations

Fixes #64

Author: epage

CONTRIBUTING.md

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+Thanks for wanting to contribute!
+
+Feel free to create issues or make pull requests, we'll try to quickly review them.
+
+If you need to reach out to us, find a relevant [issue](https://github.com/killercup/assert_cli/issues) or open a new one.
+
+# Issues
+
+Some helpful pieces of information when reporting issues
+* assert_cli version
+* rust version
+* OS and version
+
+# Pull Requests
+
+## Project Ideas
+
+If you're looking for things to do check out the [open issues](https://github.com/killercup`/assert_cli/issues).
+Or take a grep through [all TODO comments](https://github.com/killercup/assert_cli/search?q=TODO) in the code and feel free to help us out there!
+
+## Best Practices
+
+We appreciate your help as-is.  We'd love to help you through the process for contributing.  We have some suggestions to help make things go more smoothly.
+
+Before spending too much time on a PR, consider opening an issue so we can make sure we're aligned on how the problem should be solved.
+
+🌈 **Here's a checklist for the perfect pull request:**
+- [ ] Make sure existing tests still work by running `cargo test` locally.
+- [ ] Add new tests for any new feature or regression tests for bugfixes.
+- [ ] Install [Clippy](https://github.com/Manishearth/rust-clippy) and run `rustup run nightly cargo clippy` to catch common mistakes (will be checked by Travis)
+
+# Releasing
+
+When we're ready to release, a project owner should do the following
+- Determine what the next version is, according to semver
+- Update the version in `Cargo.toml` and in the `README.md` and commit
+- Run `git tag v<X>.<Y>.<Z>`
+- Push all of this to `master`
+- Create a github release
+  - Identify what fixes, features, and breaking changes are in the release.
+- Run `cargo publish` (run `cargo login` first if needed)