Add CONTRIBUTING.md

Author: maneatingape

CONTRIBUTING.md

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+# Contributing
+
+I hope that you've enjoyed reading these solutions as much as I enjoyed writing them.
+They're pretty fast and clean...but could 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).
+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
+  or by applying the portable SIMD library.
+
+Pull requests should meet the following baseline standards:
+
+* 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` into the terminal.