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.
Actually, I think you can reduce everything from _Rationale:_ on to something like this:

> _Rationale:_ Using `let` by default precisely models the behaviour of bindings, allowing readers and the compiler to assume that symbols bound with `var` will change.