Rewrites the README to be a lot more concrete about what SourceCred is
doing, and to give more up-to-date information overall.
Test plan: The README includes a small code block with instructions for
turning on a local copy of the Cred Explorer. I ran this on my machine
(inside /tmp) and it worked locally.
This reflects our current vision that SourceCred is aimed at valuing open-source contributions via a credit graph, not on directly creating cryptotokens