Add blurb on configuration and run_mirror in README

Currently the configuration syntax is only documented in the run_mirror
source. Adding enough information to the RAEDME for users to get up and
running with a simple configuration is much more user friendly.

Change-Id: I54d978ac127dfe37348167f3a2b08cc09119619b
Merge "Add blurb on configuration and run_mirror in README"