Add editor guide

[vchuravy]

Should at a minimum include rules around conflict and recusale, as well as what to do if an editor is an author.

I propose that we allow authors to be editors, but require the following statement:

Competing interests. 

\author{} is a member of the editorial board of the JuliaCon Proceedings. The peer-review process was guided by an independent editor, and the authors have no other competing interests to declare.