docs(contributing): Add contributor documentation

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributor Guidelines
+
+The three rules that contributors MUST follow:
+
+- Discuss a feature before implementing it
+- `cargo +nightly fmt` should always be run on code before creating a commit
+- Commits should follow the [Conventional Commit] guidelines
+    - A small change in one commit is exempt from this rule
+
+Things that contributors SHOULD NOT be worried about:
+
+- Code style: `cargo fmt` renders this a nonissue
+- Code acceptance: in most circumstances, safe Rust code will be accepted on the spot
+- Commit messages: maintainers can revise them before or after merging
+
+Things that contributors SHOULD be aware of:
+
+- `rustup` should be the preferred tool for Rust developers
+- `cargo clippy` can point out the majority common mistakes in Rust code
+- `sbuild` can be used to verified that debian packages build correctly
+- `unsafe` is explicitly disallowed, unless otherwise permitted by a maintainer
+
+[conventional commit]: https://www.conventionalcommits.org/en/v1.0.0-beta.4/