Update nginx.config.sample to allow custom CSS

cf. http://meta.discourse.org/t/changing-css-in-customize-section-has-no-effect/10036

config/nginx.sample.conf

@@ -50,8 +50,11 @@ server {
       #  return 403;
       #}
 
-      # let NGINX serve images
+      # custom CSS
+      location ~ /stylesheet-cache/ { try_files $uri =404; }
+      # images
       location ~* \.(gif|png|jpg|jpeg|bmp|tif|tiff)$ { try_files $uri =404; }
+      # thumbnails & optimized images
       location ~ /_optimized/ { try_files $uri =404; }
 
       # attachments must go through the rails application to get the right content-disposition header