Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove obsolete note about the boot loader not being ready for stable.
It has been in stable for at least 5 years now.

Add $FreeBSD$

Fix a couple of nasty errors in the README.

This is a temporary README file, to help those trying to experiment
with the new boot loader configuration process. I got a lot of
help from Daniel Sobral, and both Dan and I got help from Mike
Smith. This really belongs in Warner's UPDATING, but he's not
been answering his email recently, so that will wait a little.

Robert Nordier also gave me a lot of help, but he hasn't seen the
last version, and can't be blamed for my errors.

Approved by: jkh

Reviewed by: Mike Smith