Add missing comma to README.md example (#222)

Missing comma in configuration example.

Thank you!

Author: snuggs

README.md

@@ -217,7 +217,7 @@ var atImport = require("postcss-import")
 
 postcss()
   .use(atImport({
-    path: ["src/css"]
+    path: ["src/css"],
     transform: require("css-whitespace")
   }))
   .process(cssString)