Add developer setup instructions to README

[mark-dce]

STORY
As a development team, we want clear setup instructions for the development environment, so that we can onboard new team members more easily

ACCEPTANCE

• The readme includes a short description of what the repo does
• The readme includes instructions for setting up a new development environment
• [BONUS] The instructions work and someone has tested them on a clean environment :)

NOTES
You could probably use some of the information here: https://github.com/samvera/hyrax#getting-started