Update the README once we have public docs

### Context

Right now the README looks a bit more stale. We should enrich it in the future with more links to upstream docs, once we have them and/or more sections so that users can have a better first experience and be able to:
1. Understand what the repo is about
2. How they can get started
3. What the charm is doing

### What needs to get done

1. Evaluate a small plan on what sections to include in the README
1. I.e. should we include links to our specs?
2. What other standard sections should our READMEs have?
3. Link with upstream docs to point users to more details on how to use the charm

### Definition of Done

1. The README has more context on what the charm repo is, and how users can use it