README: sync with v2.x

The README on master had grown very, very stale.  This commit copies
the README from the tip of the v2.x branch (from
#3119) and preserves a few minor
differences between master and the v2.x branch.

Signed-off-by: Jeff Squyres <[email protected]>

[skip ci]
bot:notest

Author: jsquyres