Proposed Release Schedule to end 2018

[Jonathan Riddell]

Following the meeting this week I've made a proposed release schedule
until the end of 2018

Three releases a year (5.11 October 2017, 5.12,5.13,5.14 in 2018)
Betas to happen in the week after Frameworks release
Three weeks from beta to release
5.12 to be an LTS, discussed with suse and they are fine with a
release end of Jan but I've also added in Alpha on repo freeze day
with tars made.
To accomodate this the 5.11 dates have been moved a bit, Beta on Thu
14 September 2017, release tag Thu 5 October 2017
I also added in which Frameworks version we can dep on into the schedule.

https://community.kde.org/Schedules/Plasma_5
https://calendar.google.com/calendar/embed?src=031gkgqg1hjf8lcmj0em1d2sj8%40group.calendar.google.com&ctz=Europe/London

Mistakes are easy to make in this, it's fiddly stuff, so let me know
if you spot anything or if you have other comments.

Jonathan