Update feature list language to match _s readme.

As I mentioned in Automattic/_s#533, I’ve updated the feature list to fix the differences between the main repo’s list and the list on http://underscores.me.

What I didn’t include, though, were instructions that are in the _s readme about how to implement features, because I didn’t think they were necessary on the site. Once a user has downloaded a version of _s, they probably won’t return to the website to figure out how to implement a feature, being more likely to turn to the repo itself.

If it seems necessary to include the instructions on the website, then I’ll do that, too.