add basic role documentation

defaults/main.yml

@@ -1,7 +1,20 @@
 ---
+# udev_log_level:description: >
+# The log level. Valid values are the numerical syslog priorities or their textual
+# representations: `err`, `info` and `debug`.
+# @end
 udev_log_level: info
 
+# udev_rules:description: List of udev rule objects.
+# udev_rules:example: >
+# udev_rules:
+#   - name: 70-molecule.rules
+#     content: >
+#       KERNEL=="sda",
+#       SUBSYSTEM=="block",
+#       SYMLINK+="disk/by-id/custom-device-id"
 udev_rules: []
 udev_rules_extra: []
+
 udev_rules_unmanaged:
   - 70-persistent-net