fix(README): Add String.prototype.includes to polyfill (#277)

I found the picker was failing under IE11 with the current polyfill:

`object doesn't support method or property 'includes'`

Adding String.prototype.includes seems to fix it.

README.md

@@ -190,7 +190,7 @@ Module (for bundling):
 * https://babeljs.io/docs/en/babel-polyfill
 
 Script (for script tag):
-* https://polyfill.io/v3/polyfill.min.js?features=Promise%2CSymbol%2CSymbol.iterator%2CArray.from%2CObject.assign%2CNumber.isFinite
+* https://polyfill.io/v3/polyfill.min.js?features=Promise%2CSymbol%2CSymbol.iterator%2CArray.from%2CObject.assign%2CNumber.isFinite%2CString.prototype.includes
 
 ## Development