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.
My opinion is that we aren’t, in general, expecting new users to understand the rationale in this document right away. However, once they’ve used it for a bit, they should. But, that’s just my take on it—I’d like to get the opinions of some of my colleagues on this.