Add setup and running to README

Overview

In order to be welcoming to both new programmers and those who may have never used Pelican, we should have easy setup instructions. This removes a barrier to entry for those who may be great technical writers, but not necessarily Python enthusiasts.

It will hopefully also serve as a reminder to run a local branch before submitting a merge request.

Changes

• Updates README.md with instructions for setup and running on a local branch.