Add contributing section to README
Add contributing section to README

Document how to set the project in development and how to run our tests.

This is part of
https://github.com/openSUSE/agile-team-dashboard/issues/28

freakuency/agile-team-dashboard,freakuency/agile-team-dashboard,freakuency/agile-team-dashboard,freakuency/agile-team-dashboard