Improve salespitch in README.md

Readme was a bit underselling.
- Get rid of the 'default private fork' intro paragraph, claim more of an identity.
- Refactor the readme for people ECS shopping and no familiarity with ODB.
- Strip out advanced topics of no interest to new users, now better covered by the wiki (modules, advanced weaving maven instructions).
- Add benchmark section. (Picked 16k as a typical usecase for people that need a high performance ECS)
- Order topics by relevance.
- Move CDM image down, it's giant! (I think a cutout would suffice honestly).

Preview here: https://github.com/junkdog/artemis-odb/blob/document-improved-salespitch/README.md