Rework root README.md now that site is launched

README.md at root of repo is what people currently see arriving here. We should probably revise it given that a) we now have the main site b) it is now the contribute path for devs

Suggest:

• Move current README content into e.g. notes as awesome-crypto-critique or similar (easiest)
  • Someday we can move all the library items into the library and then build the page off that but until then ... this is KISS
• Revise README with brief intro to project and instructions for devs and others (and perhaps a pointer to old awesome-cryto-critique content for people who arrive at github repo looking for that - we still have quite a few links on twitter etc to it)