Fix readme.adoc to be correct (no unnecessary root usage).

I installed Fedora 20 in a virtual image to check why @nickboldt and others
felt they needed to use root when the whole idea is not to require root for gem install.

Turns out rvm is "hard" on fedora - need to read the instructions given when running
the rvm install (using curl, not yum) which seem to vary on each OS - but main gist
is you need to run in a login shell for it to work; but the instructions for rvm explains that
when running it.

Furthermore - nokogiri and nodejs is needed on linux so added details for how to make that run.

Thus I've updated the readme.adoc to be "root" free and work properly again.