Update README

[PABourdin]

The README still strongly discurages to use this plugin on production systems.
As the last activity was over a year ago, I'm just wondering if the plugin maybe is now stable?
If yes, please update the README accordingly, so that newcomers are well informed.
Thanks a lot!

[azet]

@PABourdin

There hasn't been any significant work on this project for a couple of different reasons, all not related to the project itself but rather unavailable developer time.

[sydneyli]

Bumping up the priority of this task now that I've restarted work on it. Plugin is not stable, but a lot of the information on the README remains quite outdated.

Proposed outline:

- Initiatives
  - STARTTLS Policy List
  - Certbot plugins for MTA certificate installation

- STARTTLS Policy List
  - Usage tooling
  - Adding your domain
  - Error reporting
  - Additional fields

- Contributing 
  - Configuration generators
  - Certbot plugins
  - Testing and Feedback
    
- FAQ
  - [link to site FAQ]
  - What if things break?
  - When will this code be safe to run on a large-scale, production mailserver?

- Project timeline

I've been pulling and re-organizing related documentation files and FAQ from the website, as well as past design documents. Haven't finished the complete re-org yet, but am planning to upon return. Open to feedback and suggestions, especially relating to potentially unanswered questions and common clarification questions (for FAQ).