Update README.md

Author: creecros

Assets/css/README.md

@@ -26,3 +26,7 @@ Preset Themes that come with Customizer
 :sparkles: Material
 
 ![image](https://user-images.githubusercontent.com/26339368/49310723-1003bc80-f4ad-11e8-8c03-8390ecc78d20.png)
+
+## How to add your own css
+
+To add your own custom css files to Customizer, simply place your files in the `DATA_DIR . '/files/customizer/themes` folder. If that folder doesn't exist, create it, and then place your files in there. This will retain your custom themes through upgrades of both Kanboard or Customizer.