Add package documentation for GoDoc
Add package documentation for GoDoc

This adds a package documentation section for the semaphore package for GoDoc.
It's a transcription of parts of the `README.md` file, including the example.

Signed-off-by: Tim Heckman <8efd86fb78a56a5145ed7739dcb00c78581c5375@heckman.io>

theckman/semaphore