Added contributing guidelines.

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+# Contributing to shrug
+
+Thank you for your interest in contributing to shrug. We appreciate it!
+
+It is a good idea for you to open an issue before you start working on non-trivial features. This way we can discuss if
+the feature is desirable and how to best design and implement it.
+
+## Useful tools
+
+If you are contributing with a pull request you probably want to know about these scripts:
+
+* [`./tools/check.sh`](tools/check.sh) — Checks that everything is fine. This includes checking that everything
+  builds, the unit tests pass, and the code is correctly formatted. If you need to format the code run
+  `cargo fmt`.
+* [`cargo rdme`](https://crates.io/crates/cargo-rdme) — Updates the README with the crate’s rustdoc.