Modify README

:rotating_light: WIP, Not Ready To Merge :rotating_light:

Make significant, breaking changes to advance the `autocomplete-plus` Provider API and allow further enhancements in the future that can be backward compatible.

This PR will be done when the following are complete:

- [ ] Make `AutocompleteManager` a singleton
- [ ] Deprecate `Provider`
- [ ] Deprecate `Suggestion`
- [ ] Document acceptable properties in an anonymous object for a suggestion
- [ ] Deprecate registration functions for existing providers
- [ ] Take on dependency to [`service-hub`](https://github.com/atom/service-hub) for provider registration
- [ ] Build helper function(s) that can be passed to providers and provide default behavior (like `prefixOfSelection` in `Provider`, today)
- [ ] Evolve the options object passed to `Providers` to allow Providers to be completely decoupled from the editor (so they can be singleton also)
- [ ] Further decouple the suggestion list model from `AutocompleteManager` (introduce `SuggestionList`?)
- [ ] Rename `SelectListElement` to `SuggestionListElement`
- [ ] Co-ordinate updates to existing providers / create PRs against most popular providers where necessary 

:rotating_light: WIP, Not Ready To Merge :rotating_light: