Update 'Tell us what you Think'

Removes dead link to non-existent mailing list, adds correct Twitter link.
Hashtag idea since setting up a sub-Twitter account is prob. a no go + easy issue submission link

Greasing the skids to let people quickly get at us.
Merge pull request #47 from NoahKunin/gh-pages

Small improvements to the Tell Us What You Think docs
Add new section on endpoints
Changed to use CFPB GH style