Add webpack 5 import instructions to readme

[Yahav]

to address the following:
#239

[PaperStrike]

Good catch! 👍

[PaperStrike]

This applies to not only webpack 5, but any tool that can parse the exports field well, so it's better if you widen that description. And import it this way is actually preferred, so the description could be like:

# Pick a script in `dist` folder

## Automatic Pick

[[description]]

<script src="@sliphua/pjax"></script>

or

import Pjax from '@sliphua/pjax';

## To declare globally

…

## To import as ES module

…