add info about finding references in usemin task

[ytxiuxiu]

There is no document for how does usemin find references in files. It is important to tell users about this so that we can write task correctly to ensure usemin replace references which we want to.

[nodox]

@eddiemonge @stephanebachelier @sindresorhus Seems good to me. Can anybody sign off before I merge?

[eddiemonge]

The language is a bit confusing. More or better examples might be needed