Installation instructions

Per suggestion from @aztlan2k adding installation instructions. Also making the build status badge a link to the CircleCI dashboard for the project.

@ustudio/dev Please review.
Actually, yes. In general, we probably should be updating the README file with every release, anyway (to document what we changed, unless it's just a bug fix), and I find it more convenient when projects list it on their README file, so I don't have to then go to PyPi to figure out what the latest stable is.