Add contributing instructions

Author: maneatingape

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contributing
+
+I hope that you've enjoyed reading these solutions as much as I enjoyed writing them.
+They're pretty fast and clean...but can you make them even *faster and cleaner*?
+
+If you have thought of an improvement then please
+[open a pull request](https://github.com/maneatingape/advent-of-code-rust/compare).
+Contributions are encouraged and valued. ❤️
+
+
+It should generally fall into one of the following two categories:
+
+* 🪲 **Bug Fix** Solutions try to be as general as possible but can't test for every input.
+  Your pull request fixes an incorrect answer for your input or prevents a panic.
+* 🚀 **Performance Improvement** Your pull request gives a significant (> 10%) performance
+  improvement. This could be achieved by using a better algorithm, better low level optimizations,
+  by applying the portable SIMD library or by any other means!
+
+## Legal Notice
+
+When contributing to this project, you must agree that you have authored 100% of the content,
+that you have the necessary rights to the content and that the content you contribute
+may be provided under the project licence.
+
+## Guidelines
+
+Pull requests should meet the following baseline standards:
+
+* Commit messages and pull request title are clear and informative.
+* Code style matches the existing code. This one is somewhat subjective but try to "fit in" by
+  using the same naming conventions. Code should be portable, avoiding any architecture
+  specific intrinsics.
+* No performance regressions. Benchmark the new solution versus the original and try to keep it
+  within a ±5% margin of error. Bug fixes have more leniency to reduce performance.
+* Tests should pass.
+* Code is formatted using `rustfmt`.
+* Code is linted using `clippy`.
+
+Formatting and linting can be executed by running [`just`](https://github.com/casey/just)
+(if installed) on the command line at the project root, or alternatively copying and pasting
+the commands from [`justfile`](justfile) into the terminal.

README.md

@@ -50,6 +50,11 @@ Place input files in `input/yearYYYY/dayDD.txt` including leading zeroes. For ex
 
 </details>
 
+## Contributing
+
+Improvements to solutions are always appreciated. Please see the
+[detailed instructions](CONTRIBUTING.md) on how to get started.
+
 ## Performance
 
 Benchmarks are measured using the built-in `cargo bench` tool run on an [Apple M2 Max][apple-link].