Provide documentation on how to contribute to documentation

[mglaman]

At the MidCamp sprints I noticed we could use a documentation guide on how to contribute to our documentation (META META)

Things:

• Point out each page has an "Edit this page" link at top
• State that they should have a GitHub account
• Show how to edit within GitHub, that it will fork and open PR, how to finalize the creation of PR (it's a three step process.)
• Provide links to ReST basics
• State that images cannot be uploaded from main repo
  • Check if images can be uploaded in their own fork from the UI
  • State they'll have to clone locally and edit to change images / update them
• Verify contributors can add new pages via the UI
• State heavy additions might be best done by cloning down the repository

[AndyD328]

Working on this, should have something in the next few days. I'll make a pr of how far I get if it's not complete then.