Guide: pointers

This is the next section of the guide, and it's on pointers. It's not done yet, as I need to write the section on ownership and borrowing, but I figured I'd share the rest now, to get feedback on the rest of it while I take some time to get that right.
This should say earlier on that references are immutable. Right now it's not immediately obvious that they're different from C pointers. (it says "Because references are immutable" later on)