Make instructions for adding a build button to TeXShop

[rbeezer]

Perhaps this could go in the Author's Guide somewhere, parallel to the Windows-specific instructions. Part of doing this would be testing the instructions, perhaps by somebody with limited prior knowledge. An existing example that should be similiar is at:

https://github.com/ximeraproject/static

[kcrisman]

That would be quite interesting. You can already have a Sage engine, as you know.