mention identity comparison - fixes #30 (#69)

* mention identity comparison - fixes #30

* fix identity for other types

basics/classes.md

@@ -41,6 +41,23 @@ it
 that it isn't instantiated
 - `super(..)` can be used to explicitly call the base constructor
 
+### Checking for identity
+
+For class objects, the `==` and `!=` operators compare the contents of the objects.
+Therefore, comparing against `null` is invalid, as `null` has no contents.
+The `is` compares for identity. To compare for nonidentity, use `e1 !is e2`.
+
+```d
+MyClass c;
+if (c == null)  // error
+    ...
+if (c is null)  // ok
+    ...
+```
+
+For `struct` objects all bits are compared,
+for other operand types, identity is the same as equality.
+
 ### In-depth
 
 - [Classes in _Programming in D_](http://ddili.org/ders/d.en/class.html)