diff --git a/README.md b/README.md
index 9a13e72..a25d601 100644
--- a/README.md
+++ b/README.md
@@ -162,6 +162,9 @@ For adding your own rule you should create `your-rule-name.py`
 in `~/.thefuck/rules`. Rule should contain two functions:
 `match(command: Command, settings: Settings) -> bool`
 and `get_new_command(command: Command, settings: Settings) -> str`.
+Also the rule can contain optional function
+`side_effect(command: Command, settings: Settings) -> None` and
+optional boolean `enabled_by_default`
 
 `Command` has three attributes: `script`, `stdout` and `stderr`.
 
@@ -177,6 +180,12 @@ def match(command, settings):
 
 def get_new_command(command, settings):
     return 'sudo {}'.format(command.script)
+    
+# Optional:
+enabled_by_default = True
+
+def side_effect(command, settings):
+    subprocess.call('chmod 777 .', shell=True)
 ```
 
 [More examples of rules](https://github.com/nvbn/thefuck/tree/master/thefuck/rules),