Update installation documentation

The current installation documentation didn't get me very far in getting set up, in part because it seems outdated. I added a document with the commands I ran on my MBP to get Zipkin working and cleaned up some of the explanation around it. NOTE: this is not quite ready for merging as there are a few TODOs that need review where I wasn't sure if some steps were still required.

The next step is to update the website (in the gh-pages branch) to reflect these changes; or we might decide that the master branch is the Source of Truth and we should just point the website to /docs for installation instructions.