Add CONTRIBUTING.md

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

CONTRIBUTING.md

+# Contributing
+
+Patches should be sent to the [aur-dev@archlinux.org][1] mailing list.
+
+Before sending patched you are recomended to run the `flake8` and `isort`.
+
+You can add git hook to do this by installing `python-pre-install` and running
+`pre-install install`.
+
+[1] https://lists.archlinux.org/listinfo/aur-dev