knobs file and porters-handbook update.

[Michael Johnson]

Hey
     I'm planning on committing an update to doc/en_US.ISO8859-1/ 
books/porters-handbook/book.sgml
which will add info about a new ports/KNOBS file, which includes a  
list of popular knob-names and
knob-descriptions.

I'm not the best writer and I was hoping you guys would look at
http://people.freebsd.org/~ahze/porters-handbook.diff
and
http://people.freebsd.org/~ahze/KNOBS

and critique it before I commit.

Michael