add a jsnext:main so we can distribute ES6 modules

[Whoaa512]

Also add modules to the distribution files and relevant docs to README and CHANGES.

See https://github.com/rollup/rollup#can-i-distribute-my-package-as-an-es6-module

[taion]

I don't think this is a good idea.

• Our test coverage runs against the Babel transpiled files, so we don't actually verify functionality when using e.g. rollup directly
• The shipped npm package only includes lib/ and doesn't include modules/ at all
• The source uses ES7 features like class property initializers and would still need to go through Babel again anyway, so it's not sufficient to just use a ES6-aware bundler
• rollup has minimal adoption next to e.g. Browserify and webpack and I think pointing to it in our README is just going to confuse users

[taion]

We do have to resolve this to let users take full advantage of webpack 2.0, but I'm not sure this PR is a good way forward.

[taion]

As it stands, I think if we wanted to do this, we'd have to ship e.g. a es6/ build that transpiles everything except for the module system. It's not a good idea to ship the raw source instead of the transpiled source; packages in general should not make users deal with transpiling their code.

Additionally, I'd much rather point users at webpack v2.0.0-beta rather than rollup, just because webpack has a lot more adoption and people who otherwise haven't already made a choice of package bundler would likely be quite a lot better served by going there, in terms of e.g. their ability to get help or find examples or documentation.

I'm going to close this PR for now and open a new issue for discussion. I like the idea on paper of shipping an ES6 module build that allows users to both import from react-router and not include modules that they don't use, but just exporting our raw source is not the correct way to do this.

[taion]

We'd appreciate hearing your thoughts on #2524. If we do decide to go with exposing modules/, I'd be happy to re-open this PR, but I think there are better options available toward this end. It'd be good to get some discussion before just adding another build target, though.