Readme fixes

Hi Tor team,

In preparation for use on Measurementlab.net, I made some changes to the installation instructions in the readme. I took out references to outdated code based on feedback from art@torproject.org, and made fixes and readability changes based on my own testing. Note that everything below "Using Ooniprobe" is mostly untouched.

I'll look out for any comments or corrections and happily incorporate them.  Thanks!