Expand and simplify the Migrillian README doc

[AlCutter]

https://github.com/google/certificate-transparency-go/tree/master/trillian/migrillian

Locally copying the contents of CT logs for experimentation and playing is a thing which comes up quite frequently, and we often cite Migrillian as one mechanism. However, the current docs are quite sparse and are very technical from the offset (talking about PREORDERED_LOGS, etc.).

It'd be nice to have some more accessible introductory docs and a perhaps HOWTO+Dockers images, etc. to help folks get started.

[pav-kv]

Add documentation on how to use Migrillian for:

• migrating CT logs to Trillian
• operating CT log mirrors