Consolidate contribution instructions

[hauten]

• Maybe a new Contribute page?
• Look at the various READMEs and see if the info can live elsewhere (consolidate/delete all but the top-level README)
• Update FAQ accordingly
• Include list of which FA icons are used for categories
• Per Doug: any menu changes will need to be made manually in the includes/header.html files instead of by adding each page to the menu using page attributes
• Git rid of notes.md in favor of this new page or area of the site