Doc: specify that a polyfill for Object.assign is needed #56

Old browsers (IE 11, Safari on iOS 8) do not support Object.assign but it is used in typesafe-actions. Many other libraries are using a polyfill (short custom implementation or external polyfill bundled using Babel). As this package targets es5 (in tsconfig.json), Object.assign is not transpiled and generate errors in those old browsers.

I am fine with this package targeting es5, but it should be noted explicitely in the README, and a list of needed polyfills should be provided. For example, here is how React does it: https://reactjs.org/docs/javascript-environment-requirements.html

I tested my app with Object.assign, Map, Set and Symbol polyfills (the 3 latest needed for React) and it worked fine on iOS 8 & IE 11, so I guess Object.assign is the only one needed (or maybe Symbol too?) for typesafe-actions.