README: Update text for post-acmev2 world. (#116)
README: Update text for post-acmev2 world. (#116)

This commit updates the README now that Boulder supports ACMEv2 and
we're using Pebble slightly differently than was described previously.
This commit also adds text describing the `-strict` flag.
letsencrypt/pebble,letsencrypt/pebble,shred/pebble,letsencrypt/pebble,shred/pebble