[tz] Notice Lead Time Tracking

[Paul Eggert]

Brian Inglis wrote:
> Given some recent last minute end of year announcements, could I please suggest
> creating a new document to formally track metadata about every time change

Something like that might be nice, but I'm afraid I don't have the time to work 
on it myself. An effort like that might start with a prototype for the data for 
2018, as pretty much all the information is public, either in the tz mailing 
list archives, or cited in URLs to dated pages.