browser build does not work out of the box for webpack

This isa:

- [X] Bug Report

Which concerns:

- [X] flow-runtime
- [X] The documentation website

---
I am not sure about rollup, but using flow-runtime in browser, compiled with webpack does not work out of the box. The user has to either:
- `import 'regenerator-runtime'` at the entrypoint
- webpack config `resolver.alias: { 'flow-runtime': 'flow-runtime/dist/flow-runtime.es2015.js' }`
otherwise the browser throws the following error.
```
Uncaught Error: Cannot find module 'regenerator-runtime'
at webpackMissingModule (webpack:///./node_modules/flow-runtime/dist/flow-runtime.umd.js?:13)
```
I have found the issue #29 which created a separate browser bundle, though it appears `regenerator-runtime` was originally part of that bundle. If this separation intentional, then perhaps the documentation should be updated to explain how to fix this. Otherwise, `regenerator-runtime` is not being added to the browser bundle like it should be.