[Fix #495] Add some ClojureScript usage documentation.

This doesn't go into too much detail of how to set up a CLJS environment, since this is covered well by the documentation for Austin/Weasel themselves. I think one other useful addition would be minimal known-to-work versions of CLJS/Piggieback/Austin/Weasel, but this is complicated slightly by the fact that different versions of Austin and Weasel have different minimum CLJS versions (CLJS releases do not use SemVer and move pretty fast). Thoughts?
Btw, it seems to me this would fit best under `Basic Usage`.