README instructions might be outdated with the current `reveal.js`

[amtoine]

Hello there 👋 😋

The problem

just to let you know that adding

{
    src: 'plugin/external/external.js',
    condition: function() { return !!document.querySelector( '[data-external]' ); }
},

to Reveal.initialize inside index.html does not appear to work for me 🤔

A possible fix

instead of changing the Reveal.initialize call, i have added

<script src="plugin/external/external/external.js"></script>

right before Reveal.initialize and that works 👌

Another thing

i also find that running git submodule add https://github.com/calevans/external plugin/external and using plugin/external/external/external.js as the path to the script instead of plugin/external/external.js is a bit simpler to not make any error in the paths 😋

cheers, really nice project 👋