Zenodo reference

**Current Problem**
When writing some papers related to usage of D-Wave Ocean, I would like to add a reference to the software. Unfortunately there is none. Other software stacks to access Quantum COmputing resources (e.g., Cirq or Qiskit) count with such a reference.

**Proposed Solution**
Add a Zenodo doi such that people can cite `dwave-ocean` when they use it and maybe add something to the README adding instructions on how to cite your work.

**Alternatives Considered**
Zenodo seems to be the most natural way of doing this, but there are alternatives such as Journals of Open-source Software.