Add 'vanilla' distribution section to readme.

The goal of this pull request is to increase clarity for new developers. Currently, all of the documentation and automated processes assume that the reader wants to build Semantic and customize it as they go, but the average developer is going to want to just download something prefabricated and play with it before committing to setting up the build process.

As a result, it is not even clear that the latest version of Semantic itself is usable without going through the build process; it wasn't until I cloned the repo, ran the build process, and saw in my `git status` output that there were no new files that I finally realized `dist/semantic.{js,css}` already existed beforehand.

This is one of those things that is flagrantly simple and obvious once you understand it, but I lost a day figuring this out, and there's no reason not to clarify this in the main readme.