Add comment to FxRController that describes our bootstrapping process

[patrickoshaughnessey]

From @thomasmo, in PR code review: "so...somewhere, i think we should have a load-bearing comment that explains our bootstrapping process. it was never easy, and now we made it harder (even if it's for good reasons).
i don't think the comment has to be super involved, but it should include all the major threads (or processes), callbacks, and/or state involved from startup to the browser window visible in the 3d environment.
here's an example of how i did it in fx: https://searchfox.org/mozilla-central/source/gfx/vr/nsFxrCommandLineHandler.cpp#32"

[patrickoshaughnessey]

PR #189