Reword README regarding environment variables

Reword README regarding environment variables

This clarifies the usage somewhat, without needing changes to existing
functionality. I may look into reworking the variables, but until then
this will be an improvement. That is to say, this is meant to NOT
resolve issue #230.

- Add section heading for variables
- Move `predetect`/`sleep` example up above new section
- Add example usage of variable, and explain the oddity with 'current'
profile during preswitch.