Clarify "member" vs "affiliate"

[jducoeur]

On the Projects page, we have splufty new icons distinguishing each Project as being either "typelevel member" or "typelevel affiliate". At the top or bottom of the page, we should have brief descriptions of what that means, concisely and reasonably clearly. (These may or may not link to the Charter or more in-depth discussions, but initially let's just give people some idea what they are looking at.)