Update the man page
Simplify installation instructions

Focus on the Go version and remove instructions for Ruby version
Don't suggest piping `pull-request | pbcopy` because of vim

With an interactive editor popping up during `pull-request`, it's not
advisable to redirect stdout to somewhere other than the terminal
because vim will complain. However, use this example to demonstrate how
you can skip the editor if you've already prepared the pull request
message in a file (in a corporate environment, likely with some
automated scripts).
It's not just me maintaining anymore