[ZIP 0] Document changes in the process for draft ZIPs

[daira]

• Say that draft ZIPs go in the zips/ subdirectory.
• We now merge drafts before assigning numbers. (Change the second-to-last para of the ZIPs workflow to say that a ZIP number won't necessarily be assigned at this point.)
• "ZIP Editor Responsibilities" may need rewording; it should also apply to new ZIP drafts.
• Similarly for the second-to-last para of "Reviewing a ZIP".