update docs to enable gallery with data-featherlight="gallery"

[leph83]

What kind of issue is this? (put 'x' between the square brackets)

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I've used the wordpress plugin and wanted use featherlight on custom code.
The docs is really good, I figured how to use featherlight on one image.
By try and error i figured that I had to add data-featherlight="gallery" to the container of the images to enable
the gallery function. Would be nice if you added that to the docs.