Add a guide to contributing

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+Install [the dependencies](https://github.com/kovidgoyal/kitty#dependencies)
+using your favorite package manager. Build and run kitty [from
+source](https://github.com/kovidgoyal/kitty#install-and-run-from-source).
+
+Make a fork, submit your Pull Request. If it's a large/controversial change, open an issue
+beforehand to discuss it, so that you don't waste your time making a pull
+request that gets rejected.
+
+If the code you are submitting is reasonably easily testable, please contribute
+tests as well (see the `kitty_tests/` sub-directory for existing tests, which
+can be run with `./test.py`).
+
+That's it.