create a section plugging use of github and best practices for it

[gbinal]

include notes like https://pages.18f.gov/open-source-program/github-notes/

[gbinal]

More material to factor in -

• https://github.com/government/best-practices/issues/68
• https://github.com/government/best-practices/blob/master/docs/security.md#certifications
• https://github.com/government/best-practices/issues/35
• https://github.com/government/best-practices/issues/38
• https://github.com/government/best-practices/issues/36
• https://github.com/github/government.github.com/pull/258
• https://github.com/government/welcome
• Link to major agencies using github