Updated README to reduce duplication
Removed configuration options from the README, standalone mode and moved to docs.
Remove ZK mention.
Remove redundant note
Add note on Docker.
Development stuff in README, production stuff in docs
Add note on backing up ZooKeeper.
Grammar.
Add backup note to upgrade page.
Capitalization
Merge pull request #550 from mesosphere/readme-edits

Updated README to reduce duplication. #432