README: add Contributing section.

Also, explicitly welcome new/beginner contributors and give them a suggested
starting point to contribute to Homebrew. Based on the boiler plate I tend to
send people to for GSoC, Outreachy or generally people who say "I want to try
and contribute to open-source".

README.md

@@ -16,6 +16,13 @@ Second, read the [Troubleshooting Checklist](https://github.com/Homebrew/brew/bl
 
 **If you don't read these it will take us far longer to help you with your problem.**
 
+## Contributing
+We'd love you to contribute to Homebrew. First, please read our [Contribution Guide](https://github.com/Homebrew/brew/blob/master/.github/CONTRIBUTING.md) and [Code of Conduct](https://github.com/Homebrew/brew/blob/master/CODEOFCONDUCT.md#code-of-conduct).
+
+We explicitly welcome contributions from people who have never contributed to open-source before: we were all beginners once! We can help build on a partially working pull request with the aim of getting it merged. We are also actively seeking to diversify our contributors and especially welcome contributions from women from all backgrounds and people of colour.
+
+A good starting point for contributing is running `brew audit` (or `brew audit --strict`) with some of the packages you use (e.g. `brew audit wget` if you use `wget`) and then read through the warnings, try to fix them until `brew audit` shows no results and [submit a pull request](https://github.com/Homebrew/brew/blob/master/share/doc/homebrew/How-To-Open-a-Homebrew-Pull-Request-(and-get-it-merged).md). If no formulae you use have warnings you can run `brew audit` without arguments to have it run on all packages and pick one. Good luck!
+
 ## Security
 Please report security issues to [email protected].