Optionally pull repo readme text if no description provided in civic.json

When reading the projects listing, if no project description was provided, the viewer is left wondering what the thing is all about.  If the github repo has a readme.md, it's contents can be imported as the project description - or at least the first 300 words thereof, to save having to click through to the repo (and non-technical readers won't thing to do so anyway). 
