Improved contributing guidelines.

CONTRIBUTING.md

@@ -1,13 +1,17 @@
 # Contributing to rpds
 
-Thank you for your interest in contributing to rpds.  We appreciate it!
+Thank you for your interest in contributing to rpds. We appreciate it!
 
-If you are contributing with a pull request you might want to know about a few scripts:
+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.
 
-* [`./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
+## 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`.
-* [`./tools/codecov.sh`](tools/codecov.sh) — Creates a code coverage report.  There is not a strict code coverage
+* [`./tools/codecov.sh`](tools/codecov.sh) — Creates a code coverage report. There is not a strict code coverage
   threshold, but we do want pretty much everything tested.
-* [`./tools/update-readme.sh`](tools/update-readme.sh) — The [`README.md`](README.md) is auto-generated based on the
-  crate’s documentation in [`src/lib.rs`](src/lib.rs).  To update it run this script.
+* [`cargo rdme`](https://crates.io/crates/cargo-rdme) — Updates the README with the crate’s rustdoc.