Added inheritance/extension doc

Author: C-Stevens

doc/inheritance-extension.md

@@ -0,0 +1,36 @@
+[<-- Return to index](../README.md)
+# Inheritance/Extension
+
+### How are classes inherited/extended?
+#### PHP
+Classes are extended by use of the `extends` keyword.
+```php
+<?php
+	class myClass {
+    	private $foo;
+        public function setFoo($value) {
+        	$this->foo = $value;
+        }
+    }
+    
+    class myOtherClass extends myClass {
+    	public $bar;
+        public function baz() {
+        	print "This is a unique method that myClass doesn't have";
+        }
+    }
+?>
+```
+#### Python
+Classes are inherited as they are defined:
+```python
+class myClass:
+	foo = 1
+    def setFoo(value):
+    	self.foo = value
+
+class myOtherClass(myClass):
+	bar = "apples"
+    def baz():
+    	print "This method is unique to myOtherClass and isn't in myClass"
+```