document (and upgrade) windows support

[ctb]

turns out we've supported windows for a little while, which I'd forgotten!

So people have been downloading sourmash-minimal on conda-forge (which supports Windows builds) and then installing extra deps themselves!

Interesting side note, we're getting more downloads of sourmash-minimal on Windows than on Mac OS X.

@luizirber is almost finished with https://github.com/conda-forge/sourmash-minimal-feedstock/pull/26, which is adding in the rest of the dependencies from sourmash into sourmash-minimal; this will make sourmash-minimal a full sourmash package, with full? Windows support.

So I think we should:

• [ ] add documentation to ...somewhere about our Windows supprot.
• [ ] have someone (presumably me, with my fancy Windows VM) test out sourmash install and execution to verify that it actually works well