As discussed at ISC all hands shortened README a lot.

Reduced by almost 200 lines.

Mention DHCP echo server.

Point to guide in txt format (now in repo).

Removed some redundant details that are in guide.

NOTE: This removed details about 1) gtest / make check 2) coverage
testing; 3) run_*.sh ; 4) running (already in guide); 5) bindctl
config examples.

I will make sure these are documented in guide, in the wiki,
or at least re-added to the doc/ directory. But some may not be
in this week's snapshot tarball.

I opened one ticket to handle the bindctl config examples (#1302).

This was discussed a little on jabber.