Doc for screenshot and let us specify destination

SCREENSHOT_FAILURES was left undocumented, add a very basic
documentation in README.md.

The new SCREENSHOT_FAILURES_PATH lets one override the destination
directory for screenshots (untested).

Change-Id: I2e659aef148576a8a49c3940b7da44ef7a9a6d45
Merge "Doc for screenshot and let us specify destination"