mini-tutorial: gothic/equilateral Arches made easy

[VladD]

Thanks guys.

@pp: true, I did use the 3 cylinder as a starting point in my first tests, but these kind of arches are most often modeled into already existing meshes, where it would be less convenient to fit in another model - hence the workflow I opted for, and shown. Unless, perhaps, one is making a medieval game model, since those often have windows as extra meshes just slapped on building walls.