Docs on how to setup leaflet for contributions. #2

I spent some time working through getting leaflet set up and running locally and documented the process. I've put the notes in the README though it might be better placed in a CONTRIBUTIONS doc (or even its own thing in a docs folder). The main editor functionality is all working and everything seems connected up correctly; I am still unsure how exactly to get the publishing info working (though I did manage to stand up the feed service).

Would love some pointers/clarification and then I can clean this doc up for you and maybe do a bit of hacking myself.