Centralized README for documentation contributors

[moxiegirl]

We should have a central readme for documentation contributors and then included it in every project/docs. README should cover these topics which we all agree on:

• 80 character wrap is standard across the projects
• Gists process is preferred over line-by-line commentary
• After Gist round if a user just cut-n-paste but no formatting: pull and carry formatting fix
• Editing directly in Git while easy to do; follow on changes can quickly overwhelm the newbie user who is gravitates to it (Thea)