Add documentation for if-else from PR#213

[schoen]

This documents the upcoming if-else feature in SimplicityHL.

Pending merge of BlockstreamResearch/SimplicityHL#213.

[imaginator]

This is open for 5 months. Can we close or commit please. This sounds like critical information to have in the site.

[schoen]

The functionality that this documents (BlockstreamResearch/SimplicityHL#213) hasn't actually landed in the SimplicityHL project yet.

@stringhandler @apoelstra Any prognosis on when the if-else functionality might land in the language? I haven't looked at the architectural details but this particular feature feels to me like it's not that bound up with complicated type theory issues (for example, there's a very straightforward deterministic translation between this and a match on a bool, and I expect there should continue to be such a translation even if our type system gets richer and more complex).