Add PLEP workflow to README

Add a PLEP workflow to the README.rst.  This should (at least) cover...

1. How to start an upload to Zenodo to reserve a DOI.
1. How to fill out the Zenodo metadata.
1. Guideline for getting committee approvals.
    *  It's probably best to do this by getting approvals on the PR, then we have an easily discoverable track record.
1. Finishing Zenodo upload after PLEP acceptance.