feat: add CONTRIBUTING.md (#57)

* feat: add CONTRIBUTING.md

* chore: shorten CONTRIBUTING doc

---------

Co-authored-by: Jonathan Wang <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,62 @@
+## Contributing to halo2-lib
+
+Thanks for your interest in improving halo2-lib!
+
+The [discussion Telegram][dev-tg] is available for any questions you may have that are not covered in this guide.
+
+### Code of Conduct
+
+This project adheres to the [Rust Code of Conduct][rust-coc].
+
+### Ways to contribute
+
+There are fundamentally four ways an individual can contribute:
+
+1. **By opening an issue:** For example, if you believe that you have uncovered a bug,
+   creating a new issue in the issue tracker is the way to report it.
+2. **By adding context:** Providing additional context to existing issues,
+   such as screenshots, code snippets and helps resolve issues.
+3. **By resolving issues:** Typically this is done in the form of either
+   demonstrating that the issue reported is not a problem after all, or more often,
+   by opening a pull request that fixes the underlying problem, in a concrete and
+   reviewable manner.
+
+### Submitting a bug report
+
+When filing a new bug report in the issue tracker, you will be presented with a basic form to fill out.
+
+The most important pieces of information we need in a bug report are:
+
+- The branch you are on (and that it is up to date)
+- The platform you are on (Windows, macOS, an M1 Mac or Linux)
+- Code snippets if this is happening in relation to testing or building code
+- Concrete steps to reproduce the bug
+
+In order to rule out the possibility of the bug being in your project, the code snippets should be as minimal
+as possible. It is better if you can reproduce the bug with a small snippet as opposed to an entire project!
+
+### Pull request guidelines
+
+Please file a pull request to resolve an issue.
+
+#### Adding tests
+
+If the change being proposed alters code, the pull request should include one or more tests to ensure that halo2-lib does not regress in the future.
+
+Types of tests include:
+
+- **Positive unit tests**: Checking your ZK circuit produces the expected answer, compared to a native Rust implementation of the same function. This can use either `MockProver` or the real `Prover` and `Verifier`.
+- **Negative unit tests**: Checking that your ZK circuit is not under-constrained: the circuit should _not_ verify/pass if given witness values that are deemed incorrect. This can use either `MockProver` (for test execution speed) or the real `Prover` and `Verifier` (for ultimate security).
+
+The easiest way to get started with tests is to look at existing examples and replicate them.
+
+#### Opening the pull request
+
+Before opening a pull request, please make sure all code in the repo compiles, and clippy warnings are resolved.
+
+From within GitHub, opening a new pull request will present you with a template that should be filled out. **Please make pull requests to the [community-edition](https://github.com/axiom-crypto/halo2-lib/tree/community-edition) branch.**
+
+A maintainer will review your pull request and provide feedback and possibly request changes.
+
+[rust-coc]: https://github.com/rust-lang/rust/blob/master/CODE_OF_CONDUCT.md
+[dev-tg]: https://t.me/halo2lib