Add a basic CONTRIBUTING file (#77)

* Add a basic CONTRIBUTING file

* Mild updates to CONTRIBUTING, mainly about formatting

* Add import ordering to CONTRIBUTING

CONTRIBUTING.md

@@ -0,0 +1,36 @@
+# Contributing
+
+Contributions come in a variety of forms. Developing Serai, helping document it,
+using its libraries in another project, using and testing it, and simply sharing
+it are all valuable ways of contributing.
+
+This document will specifically focus on contributions to this repository in the
+form of code and documentation.
+
+### Rules
+
+- Stable native Rust, nightly wasm and tools.
+- `cargo fmt` must be used.
+- `cargo clippy` must pass, except for the ignored rules (`type_complexity` and
+`dead_code`).
+- The CI must pass.
+
+- Only use uppercase variable names when relevant to cryptography.
+
+- Put a space after comment markers.
+- Don't use multiple newlines between sections of code.
+- Have a newline before EOF.
+
+### Guidelines
+
+- Sort inputs as core, std, third party, and then Serai.
+- Comment code reasonably.
+- Include tests for new features.
+- Sign commits.
+
+### Submission
+
+All submissions should be through GitHub. Contributions to a crate will be
+licensed according to the crate's existing license, with the crate's copyright
+holders (distinct from authors) having the right to re-license the crate via a
+unanimous decision.