ANGLE documentation

[Frosty-J]

Fed up of referring to libgdx/libgdx#6672 - it is good for improving compatibility on Windows and quite possibly macOS, though not without issues. This puts it in a state where, while I think it's worth using in games, it is also worth being aware of its caveats which seem unlikely to get fixed (since it stopped receiving changes less than a week after merging). My only hope is Simon might be able to maintain it, since he made libgdx/gdx-angle-natives#5, but that's a bit much to ask.

I have not decided where it would go on the wiki, but it would be good to put all information surrounding it in one place and so that anyone browsing the libGDX website can learn of its existence. As, outside of our little bubble, gdx-angle is relatively unknown.