Add a CONTRIBUTING.adoc file to the repository

[Jane Sandberg]

Many github repos have a CONTRIBUTING file in the root directory, with helpful information about how to create a development instance, format a pull request, where to find code guidelines, and where to find additional help.

Github displays these CONTRIBUTING files in helpful ways throughout its interface. For example, when a new contributor is opening their first pull request (e.g. https://github.com/openshift/origin/pulls), Github shows a link to the contributor guidelines.

Note that we could totally use asciidoc for this, rather than markdown.

Adding a CONTRIBUTING file is a common recommendation on lists of how to encourage new contributors (e.g. https://medium.com/free-code-camp/how-to-attract-new-contributors-to-your-open-source-project-46f8b791d787)