Add tagged releases to match godot releases about godot-demo-projects

[akien-mga]

That's up for discussion, but I chose not to do so as I believe the demos are not strongly linked to the Godot version (especially not to patch releases like 2.0.3 or 2.1.1).

So instead I chose to create a branch for the current stable version: https://github.com/godotengine/godot-demo-projects/tree/2.1 (currently the same as master)
New demos that would be 2.2-specific (e.g. showcasing VisualScript or the advanced networking API) would go in the master branch only, while bug fixes on existing demos or new improved demos would go in both branches.

The versioned demos available from godotengine.org are therefore not tag-bound, but a snapshot of the 2.1 branch at a given date, see https://downloads.tuxfamily.org/godotengine/demos/2.1/

My intent is that people who downloaded the demos for 2.1 won't have to redownload them for 2.1.1 or 2.1.2, but that people downloading demos 2 months from now would get the latest version from the stable branch.

It's slightly less convenient for Linux packagers, but a snapshot of the 2.1 branch can easily be retrieved with https://github.com/godotengine/godot-demo-projects/archive/2.1/godot-demo-projects-2.1.tar.gz (which is what I use in my Mageia package: http://svnweb.mageia.org/packages/cauldron/godot/current/SPECS/godot.spec?view=markup)

As I mentioned, I'm open for discussion, so if there are strong arguments in favour of tagging releases instead of using this stable branch approach I went for, I might reconsider :)

[sambler]

I seem to be the only one thinking this so closing.