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

[marfarma]

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).