README.md: update and freshen
README.md: update and freshen

Also move the more important developer subsection (Running Codebase
Checks) to the top.

RJPercival/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go