Document how to troubleshoot .rsic issues

[perryprog]

We should document somewhere that you should (almost always) be using SpriteSpecifier.Rsi instead of SpriteSpecifier.Texture to refer to a png, and that you can troubleshoot issues with SpriteSpecifier.Textures not showing up in production by connecting to your local server from the launcher.

See also space-wizards/space-station-14#39461.