Updates to Sheer settings, jump link icons, README

- Updates to work with new Sheer indexing methods
- Fixes appearance of icons in jump links that break onto two lines
- Adds info on setting up GovDelivery support and how we version this repo to the README

@himedlooff or @marcesher, Could you please confirm that I captured the outcome of our versioning discussion correctly?
It might be worth adding a comment that indicates which icon this refers to.