Clean up the readme

I love when.js, but I cringe every time I have to go check out the readme/docs because I know I have to scroll through a boatload of version changelogs before I get there. You guys maintain a changelog file already, there's no reason to duplicate it in the main readme, you can just link to it for those who are interested.

While I was in there working on the readme, I made a few more edits to make the headlines more clear and consistent across the document.

Much :sparkling_heart: for a fantastic library, and hoping you guys are ok with these changes because it would make my (and I think many others') lives much happier every time we go searching for the docs.