add requirements in README

README.md

@@ -21,6 +21,7 @@ on Debian Stretch (9) with sudo:
     sudo apt install python3 python3-venv python3-dev python3-pip
     sudo apt install redis-server
     sudo apt install zlib1g-dev libjpeg-dev
+    sudo apt install imagemagick libmagickwand-dev ghostscript
 
 for better preview support: