[PROJ] Release schedule between version 6.0.0 and 7.0.0

[Greg Troxel]

[prematurely sent the previous]

Thanks - that all sounds good.

Certainly thinking about it later seems wise, but it's going to be
interesting to see how the transition to 6 in packaging systems goes.
While one can think about users making individual choices based on what
depending programs they want, and perhaps mulitple versions in different
prefixes or packaging approaches that basically use a prefix per package
(to avoid this issue), I think most people are going to be using what
their systems provide and thus either complaining that proj is old or
that foo is not available.

I wonder if it makes sense for proj to track the set of depending
projects that don't work with 6, with links to the tickets in those
projects' trackers.   That may be a step too far.

FWIW, postgis seems likely to have a 2.5.2 that works with proj 6 soon.
(2.5.1, the current release, does not.)