Update readme to reflect custom file locations.

[quintrino]

Hi, thanks for a great tool.

I was originally going to raise an issue about customising the location of the .fresh and .freshrc files, but looking at the perl script it looks like I can set these as ENV FRESH_RCFILE and FRESH_PATH.

Can the readme be updated listing what options can be customised?

[twe4ked]

I thought we had that but apparently it's in the Wiki not the README. I wonder if making the Wiki more prominent from the README could be a solution? Although it raises the question, why do we have the docs split into two separate places.

[jasoncodes]

My preference would be to have everything in the README and rely on the wiki for things like more detailed guides/howtos, reference documentation, “how this thing works”, etc.