Add unicode.pf2 requirement to readme.  Add instructions for fetching it.

I didn't want to commit the file here or link to it as I figured it may change often with each new release.  Hope it was okay to update the instructions to at least mention it's needed.  I couldn't think of a better option.
Make what I wrote fit in better with guide

Correct my instances of PCBSD to PC-BSD.  Show where pcbsdfont.pf2 should be copied to.