readme: revise and extend 'Getting Started' instructions
readme: revise and extend 'Getting Started' instructions

The Readme will probably undergo a more extensive rewrite, but this is a start. 

Adds instructions to migrate the database, to start cored, and to view the dashboard.

Closes chain/chainprv#1617.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain