Add "Why do my DOM nodes have two classes?" to the FAQ

This is copied from a recent chat log, we should clean this up and add it to the FAQ:

The first class (sc-bfhe32) is static per-component, meaning every instance of a <Button /> will have that class. The second is per-dynamic-stylr-fragmet, so all <Button primary /> will have the same one, all <Button secodary /> will have the same one etc. Note that the first one doesn’t actually have any styles attached to it because it’s only used for when you interpolate the component (${Button}) and so that in the DevTools you can quickly tweak all Buttons no matter if they’re primary, secondary or whatever else

[kitten]

Wow that would be an amazing addition! We’ve had to answer this too many times 😃

[nilsgoksor]

I could give this a go as my first contribution! Just to clarify, I’m supposed to make a new question in the FAQ explaining this in a good way and using an example?