added some clarifications to the dangerous/safe methods rule

README.md

@@ -570,7 +570,53 @@ syntax.
   should end in a question mark.
   (i.e. `Array#empty?`).
 * The names of potentially "dangerous" methods (i.e. methods that modify `self` or the
-  arguments, `exit!`, etc.) should end with an exclamation mark.
+  arguments, `exit!` (doesn't run the finalizers like `exit` does), etc.) should end with an exclamation mark if
+  there exists a safe version of that *dangerous* method.
+
+    ```Ruby
+    # bad - there is not matching 'safe' method
+    class Person
+      def update!
+      end
+    end
+
+    # good
+    class Person
+      def update
+      end
+    end
+
+    # good
+    class Person
+      def update!
+      end
+
+      def update
+      end
+    end
+    ```
+
+* Define the non-bang (safe) method in terms of the bang (dangerous)
+  one if possible. 
+
+    ```Ruby
+    class Array
+      def flatten_once!
+        res = []
+
+        each do |e|
+          [*e].each { |f| res << f }
+        end
+
+        replace(res)
+      end
+
+      def flatten_once
+        dup.flatten_once!
+      end
+    end
+    ```    
+
 * When using `reduce` with short blocks, name the arguments `|a, e|`
   (accumulator, element).
 * When defining binary operators, name the argument `other`.