README variable "fp" hard to use

Hi,

I have helped several coworkers with setting up bigbang installations and a couple of them hit issues when attempting to follow the instructions with the gpg key and using the ${fp} variable. I think it would be helpful to add to the comment above it that fp is the fingerprint for the gpg key. And also add that you must do the rest of the commands in the same terminal window. If you use a new window, the variable will disappear and need to be re-added. The later commands might still appear to work even if the variable is not populated or have unhelpful error messages, so I think stating this is important.