Added users-formula to README.rst

README.rst

 ---------------
 
 Permits the abitrary management of files. See pillar.example for configuration details.
+
+Overriding default values
+=========================
+
+In order to separate actual user account definitions from configuration the pillar `users-formula` was introduced:
+
+```yaml
+users:
+  myuser:
+    # stuff
+
+users-formula:
+  lookup:
+    root_group: toor
+    shell: '/bin/zsh'
+```