doc: contributor guide

[frasertweedale]

Publish an initial contributor guide that addresses the requirements
at https://github.com/freeipa/freeipa-openshift/blob/master/community-engagement-plan.md#contributor-guide.

It does not need to be complete or perfect. The acceptance criteria is that a new community member should be
able to use it to run FreeIPA in OpenShift (privileged or other caveats are acceptable) and use the instance as
a basis for testing or development.