Add setup guide for Jest and this library

[RogierdeRuijter]

In my Svelte app running test with Jest fails because the index file is of this library is of type 'mjs'.

The following error is thrown:

> jest src

 FAIL  src/App.test.js
  ● Test suite failed to run

    Cannot find module 'svelte-websocket-store' from 'src/App.svelte'

It would be nice if you could add a section to the Readme explaining how to setup Jest properly to work with this library.

[RogierdeRuijter]

This might be a difficult problem to solve since this a longer running issue as you can see here jestjs/jest#9430