adds contribution guide

CONTRIBUTING.md

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+# Contributing to rust-rocksdb
+Thank you for taking an interest in the project, and contributing to it - it's appreciated! There are several ways you can contribute:
+- [Bug Reports](#bug-reports)
+- [Feature Requests](#feature-requests)
+- [Documentation](#documentation)
+- [Discussion](#discussion)
+- [Pull Requests](#pull-requests)
+
+**Please note all contributors must adhere to the [code of conduct](code-of-conduct.md).**
+
+## Bug Reports
+[bug-reports]: #bug-reports
+- **Ensure the bug has not already been reported** - this can be done with a quick search of the [existing open issues](https://github.com/rust-rocksdb/rust-rocksdb/issues?q=is%3Aissue+is%3Aopen+).
+- **Ensure the bug applies to the Rust wrapper, and not the underlying library** - bugs in the RocksDB library should be [reported upstream](https://github.com/facebook/rocksdb/issues).
+- When [creating an issue](https://github.com/rust-rocksdb/rust-rocksdb/issues/new) please try to:
+    - **Use a clear and descriptive title** to identify the issue
+    - **Provide enough context** to acurately summarize the issue. Not every issue will need detailed steps to recreate, example code, stack traces, etc. - use your own judgment on what information would be helpful to anyone working on the issue. It's easier for someone to skim over too much context, than stop and wait for a response when context is missing.
+
+## Feature Requests
+[feature-requests]: #feature-requests
+Feature requests will primarily come in the form of ergonomics involving the Rust language, or in bringing the wrapper into parity with the library's API. Please create an issue with any relevant information.
+
+## Documentation
+[documentation]: #documentation
+Much of the documentation should mirror or reference the library's [documentation](https://github.com/facebook/rocksdb/wiki). If the wrapper or its exposed functions are missing documentation or contain inaccurate information please submit a pull request.
+
+## Discussion
+[discussion]: #discussion
+Discussion around design and development of the wrapper primarily occurs within issues and pull requests. Don't be afraid to participate if you have questions, concerns, insight, or advice.
+
+## Pull Requests
+[pull-requests]: #pull-requests
+Pull requests are welcome, and the maintainers greatly appreciate PRs that follow open-source contribution best practices:
+1. Fork this repository to your personal GitHub account.
+1. Create a branch that includes your changes, **keep changes isolated and granular**.
+1. Include any relevant documentation and/or tests. Write [documentation tests](https://doc.rust-lang.org/rustdoc/documentation-tests.html) when relevant.
+1. Apply `cargo fmt` to ensure consistent formatting.
+1. [Create a pull request](https://help.github.com/en/articles/about-pull-requests) against this repository.
+
+For pull requests that would benefit from discussion and review earlier in the development process, use a [Draft Pull Request](https://help.github.com/en/articles/about-pull-requests#draft-pull-requests).
+
+## Additional Resources
+Some useful information for working with RocksDB in Rust:
+- [RocksDB library primary site](https://rocksdb.org)
+- [RocksDB library GitHub repository](https://github.com/facebook/rocksdb)
+- [RocksDB library documentation](https://github.com/facebook/rocksdb/wiki)
+- [Rust's Foreign Function Interface (ffi)](https://doc.rust-lang.org/nomicon/ffi.html)
+