Update README.md with customservice.py

README.md

+## Prerequisite
+
+You will need the customservice.status.py file in `<salt_root_directory>/_modules`.
+
+You can get a copy of the file [here](https://gitlab.mbb.univ-montp2.fr/saltstack-formulas/check_services/-/snippets/3).
+
+
 This formula checks services/daemon from a pillar (pillar:machines/hosts\*.sls).
 
 Usage: