Readme fixes

Hi Ooni team,
In preparation for use with MeasurementLab.net, I fixed up the installation instructions in the readme. I made changes based on my own testing, as well as on help from art@torproject.org.  I took out links to outdated code and generally made it easier to read. I'm happy to incorporate any comments or changes you'd like.

Note that I only worked on the installation sections, so everything  below "Using Ooniprobe" is mostly untouched.