Add docs to the rukpak repo that explain the provisioner spec.

Acceptance Criteria:

The document should detail

• what a $PROVISIONER does (i.e. what use cases can an implementation of the spec solve?)
• why a common provisioner spec should be defined (i.e. what use cases does the spec itself solve?)
• the kinds involved (e.g. Bundle, ProvisionerClass)
• behaviorial contracts
• example implementations

The document should read linearly, introducing concepts as needed (i.e. ELI5)