Merge pull request #258 from GSI/256

clarified README regarding "css add" (refs #256)

README.md

@@ -290,8 +290,9 @@ as `KEY` and `many args` as `VALUE`.
 * `css <COMMAND>`
   - Controls CSS settings in web pages. Supported subcommands include:
     + `add <URI>`
-      * Adds a CSS file to pages when loaded. only supports one extra
-        stylesheet at a time.
+      * Adds a CSS file to pages when loaded. Despite the name, subsequent calls
+        replace any previous invocation. Meaning that only one single stylesheet
+        may be activated with this command.
     + `clear`
       Clears all user-supplied stylesheets.
 * `scheme <SCHEME> {COMMAND}`