Update readme.md to indicate that this project is a ponyfill by default

[MatthewAry]

The readme says

It's recommended to use this library in the form of a ponyfill, which doesn't inflict modifications of the global object.

The index.js file of the project does the ponyfill work already. Telling us to use the library in the form of a ponyfill is misleading, it should say something like "This library acts as a ponyfill by default, so as to not inflict modifications to the global object."

[acontreras89]

Can I ask why it is preferred to use this as a ponyfill instead of a polyfill? What are the benefits?

I was wondering but I did not want to open a new issue for such a (maybe silly) question.

cc/ @que-etc (sorry 🙏 )