Clarify that only main.less is compiled automatically.  Other .less files need to be explicitly referenced in build.sbt.  The "For example" in the current documentation makes it sound (to me) like this is one example of a general case -- anything with the extension .less will be compiled.
Merge pull request #3751 from bwbecker/LESS_doc

Clarify that only main.less is compiled automatically.  Other .less file...