Update mission statement in this repo

[dcwalk]

At our 2018 Annual Meeting we consensed on a mission statement! 🎉 🎉

Data Together empowers people to create a decentralized civic layer for the web, leveraging community, trust, and shared interest to steward data they care about.

This should be added to this repo, and previous discussion/versions should be removed (a back up copy can be added to our old notes spot on Google Drive.

TODOs:

• update first para of README.md (in Cleanup readme and update how mission statement is presented in repo #72)
• review/remove previous drafts of mission in this repo (in Cleanup readme and update how mission statement is presented in repo #72)
• review/close out of date PRs? issues? pertaining to drafting mission
• (stretch) update website!