Expand and simplify the Migrillian README doc

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.

Read the existing README and documentation under trillian/migrillian, starting with the current technical introduction and its references to PREORDERED_LOGS. Document an accessible introduction and a HOWTO for copying CT logs, including the Docker images mentioned in the issue; done means new users can follow the setup without beginning with internal terminology.

Written by the indexing model from the issue text.