Write a Hacking document section describing how to launch multiple TBBs
We should add a section to https://trac.torproject.org/projects/tor/wiki/doc/TorBrowser/Hacking that describes how to run multiple Tor Browser instances concurrently, and also how to point Tor Browser at different/system Tor instances. This is a commonly needed task for people who want to both develop Tor Browser as well as continue to use it.