apps: add description

[Stunkymonkey]

you run nix run --help, you are presented with:

      | apps.x86_64-linux.blender_2_79 = {
      |   type = "app";
      |   program = "${self.packages.x86_64-linux.blender_2_79}/bin/blender";
      |   meta.description = "Run Blender, a free and open-source 3D creation suite.";
      | };

this is currently declared optional, but missing and already shows warnings for nix flake check.

[Stunkymonkey]

ping @MatthewCroughan for review?