Add instructions for running static locally

In particular how to have static point at it's own assets, not default to
static.dev.gov.uk, which won't exist unless you're running our whole VM stack.

This makes it easier for:
 - new devs seeing how static works
 - external contributors
 - developers of apps that depend on static (eg, may use slimmer)
Merge pull request #446 from alphagov/how-to-run-locally

Add instructions for running static locally