Work on documenting usage with the less-asset-pipeline plugin.

Use with Bootstrap 3.2 currently gives this error:

| Error 2014-06-30 14:53:04,889 [http-bio-8080-exec-5] ERROR errors.GrailsExceptionResolver  - MissingMethodException occurred when processing request: [GET] /bootstraptest/assets/custom-bootstrap.css - parameters:
compile: false
No signature of method: org.mozilla.javascript.Undefined.replaceAll() is applicable for argument types: (java.lang.String, asset.pipeline.processors.CssProcessor$_process_closure1) values: [url\([\'\"]?([a-zA-Z0-9\-\_\./\@\#\?\ \&\+\%\=]+)[\'\"]?\), ...]. Stacktrace follows:
Message: No signature of method: org.mozilla.javascript.Undefined.replaceAll() is applicable for argument types: (java.lang.String, asset.pipeline.processors.CssProcessor$_process_closure1) values: [url\([\'\"]?([a-zA-Z0-9\-\_\./\@\#\?\ \&\+\%\=]+)[\'\"]?\), ...]

But 3.1 works

References #111