Add a "Here be dragons" guide to the codebase

[wetneb]

I came across this "Here be Dragons" guide to the Element code base and thought it was a nice thing to write and make available to contributors. We could have something similar for OpenRefine

[thadguidry]

Sure, but the list of "dragons we want to slay" is very, very long indeed some going back years to the very first issues still remaining open. Hmm, which maybe some of those are likely good ones to perhaps put into the "dragons we have lived with so far" since we want to slay them, but lived with them for eons? Dunno.