Explain how coverage statistics are gathered.

[bgourlie]

It would be nice if it were explained how coverage statistics were gathered. As far as I know, observatory plays some part in it, but that's about it. Ultimately, I'm trying to figure out why my package is showing zero test coverage. Knowing how this package works would be a good start to figuring that out.

Anyhow, I have an issue on my tracker here https://github.com/bgourlie/noBS/issues/3 if anyone would like to take a look at my specific setup.