Propose by-default usage of `let` over `var`.

The intent and meaning of both keywords is clear, but *let-by-default*
results in safer and clearer code.

This is simple, subtle, but IMHO important. I wasn't sure how much code sample you wanted for such a small thing, so I kept it pretty basic. If you choose to merge, feel free to edit and/or change the formatting.
Does anyone else have thoughts about this? My tendency is to let the compiler guide the way, but that is not necessarily going to be immediately obvious to first-time Swift devs.