Added example for attribute-defined-outside-init

Co-authored-by: Vladyslav Krylasov <vladyslav.krylasov@gmail.com>

Author: 2 people

doc/data/messages/a/attribute-defined-outside-init/bad.py

@@ -0,0 +1,3 @@
+class Student:
+    def register(self):
+        self.is_registered = True  # [attribute-defined-outside-init]

doc/data/messages/a/attribute-defined-outside-init/good.py

@@ -0,0 +1,6 @@
+class Student:
+    def __init__(self):
+        self.is_registered = False
+
+    def register(self):
+        self.is_registered = True