PSA: KDE CI: How-To for stable branch switches being picked up

[Friedrich W. H. Kossebau]

Hi KDE project maintainers,

tl;dr https://community.kde.org/Infrastructure/Continuous_Integration_System 
now tells how to make sure KDE CI picks up your new "stable" branch


Some days ago I found that a few products on KDE CI in the "Extragear" section 
had not seen builds of their stable branches for some months (I remember at 
least Krita & Kexi), despite being under good development.

Reason was that for those products a needed step had not been done after the 
"stable" branches were switched to new ones:
	manually triggering an initial build with the new branch
	(needed with current CI logic).

To make KDE CI less a black magic box but more a controlled tool for KDE 
developers, please find some documentation about the steps needed to be done 
for build.kde.org on switching the stable branch here:

https://community.kde.org/Infrastructure/
Continuous_Integration_System#Updating_builds_on_switching_the_.22stable.
22_branch

Already linked from https://community.kde.org/ReleasingSoftware#Branching

Please update & improve where needed (it's a wiki... ;) ).

Cheers
Friedrich