Add debug command to README

Author: harmtemolder

README.md

@@ -4,3 +4,11 @@ A Libgen Fiction store plugin for Calibre
 ## Installation
 
 Download a release from [here](https://github.com/fallaciousreasoning/CalibreLibgenStore/releases) and follow the instructions to add the plugin to Calibre.
+
+## Testing & development
+
+While working on any of the scripts, run this to update the plugin in Calibre and start it in debug mode:
+
+```shell
+calibre-customize -b . && calibre-debug -g
+```