Mention CONTRIBUTING in README

Author: Philippe-Cholet

README.md

@@ -21,10 +21,9 @@ use itertools::Itertools;
 ```
 
 ## How to contribute
+If you're not sure what to work on, try checking the [help wanted](https://github.com/rust-itertools/itertools/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22) label.
 
-- Fix a bug or implement a new thing
-- Include tests for your new feature, preferably a QuickCheck test
-- Make a Pull Request
+For more details, see the [CONTRIBUTING.md](https://github.com/rust-itertools/itertools/blob/master/CONTRIBUTING.md) file.
 
 ## License