Some fixes for README.
Some fixes for README.

- @chrisroos had already written instructions for running Solr locally
  in the Rummager README.
- SLIMMER_ASSET_HOST should be STATIC_DEV.

YOTOV-LIMITED/whitehall,alphagov/whitehall,robinwhittleton/whitehall,ggoral/whitehall,askl56/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,alphagov/whitehall,ggoral/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,alphagov/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,ggoral/whitehall,robinwhittleton/whitehall,askl56/whitehall,askl56/whitehall,ggoral/whitehall,hotvulcan/whitehall