Review did:earth method specification draft

[lohanspies]

Hi @wip-abramson @jandrieu @ig-shaun please review and comment on https://github.com/EarthProgram/did-earth/blob/draft/README.md. This is a work in progress, however, any comments or suggestions would be appreciated. Busy with the sequence diagrams, once they are completed I will update the specification accordingly.

@wip-abramson might be good for you to look at the security and privacy consideration sections as well.