add info about finding references in usemin task

README.md

@@ -346,6 +346,14 @@ The `usemin` task has 2 actions:
 * First it replaces all the blocks with a single "summary" line, pointing to a file creating by the transformation flow.
 * Then it looks for references to assets (i.e. images, scripts, ...), and tries to replace them with their revved version if it can find one on disk
 
+### Finding references
+
+`usemin` will look for different kinds of references in files of certain types. The type of the file should be indicated by using target name. That is to say, any target name in the `usemin` task should only be in `html`, `css`, `json` and `svg` so that `usemin` can lookup possible types of references in these files of a certain type.
+
+For example, if the target name is `html`, `usemin` will find references of `<img src="..."`, `<script src="..."`, `<link href="..."`, etc.
+However, `usemin` will only find references which look like `url(...)` when target name is `css`.
+For some other target names, such as `scripts`, `assets`, `usemin` will do nothing.
+
 ### Finding assets
 
 By default `usemin` will look for a map object created by [grunt-filerev](https://github.com/yeoman/grunt-filerev), located in `grunt.filerev.summary`. If it does not find it it will revert to disk lookup which is longer.