[webappsec] Draft charter for review

I've put up a draft charter based on the tentative consensus we had at TPAC:

https://github.com/w3c/webappsec/blob/master/admin/webappsec-charter-2015.html


(Maybe Mike can clue me in to how to put this on GH Pages, but I expect everyone who cares can figure out how to view a rendered version for now.)

Comments and PRs welcome!

-Brad