readme: Make a statement about hiding `node_modules` from Spotlight

[merchako]

An intuitive off-label use of Asimov would be to prevent Spotlight from search */node_modules/. Indeed, I came across Asimov while searching for a way to get Spotlight to ignore these folders. At a glance, it's unclear whether Asimov would do this. Could you add a statement to /readme to clarify whether it would?

[RobHelgeson]

I think all it would need to do to add this feature (if it does not already work) is to add a .metadata_never_index to the root of each found node_modules.