Consolidate README files

[shaycrk]

Note that the top-level README.md and sources/README.md have overlapping but different content (only the latter is actually in the served hitchhikers guide page). If we want these to overlap, can they both be sourced from the same place (I think we did something similar with triage for a while, which seemed relatively straightforward)? Otherwise, perhaps the github README should provide a link out to the guide itself but otherwise be geared towards people contributing to it?