This commit adds context-specific documentation on StorageDriver,
StorageDriverFactory, and the factory’s Register func, explaining how
the internal registration mechanism should be used.

This documentation follows from the thread starting at
https://github.com/deis/builder/pull/262/files#r56720200

.

cc/ @stevvooe

Signed-off-by: Aaron Schlesinger <aschlesinger@deis.com>