[dpub-loc] Draft update

Hi all,

I updated the markdown on the git repo (
https://github.com/w3c/dpub-pwp-loc/blob/gh-pages/locators.md), trying to
incorporate the discussion of the previous call.

I also added 2 new sections: functionalities (i.e., what specific
functionalities are needed to support PWPs, e.g., mapping from canonical to
state locator, displaying a resource, etc.), and issues.

Issue 1 is the issue as discussed last on the call, issue 2 is one Ivan
mentioned in his last comment to the previous draft: what do you get when
GETting the unpacked PWP?

Comments are most welcome!

Kind regards,
Ben