README.md: Update usage for folks running the compiled installer

With the Inno installer, there's no need to download Python or msysGit
ahead of time.  Also, folks who aren't building their own installer
can use the latest compiled release.  I'm not sure how the compilation
process works, so I'll leave that for Ethan to fill in.

Once we're actually tagging releases here, we'll probably be linking
to GitHub's releases [1] instead of files.sw-c.org, but we haven't
tagged a release yet.

[1]: https://github.com/swcarpentry/windows-installer/releases