add howto for self-referencing type hints

self-referencing-hints.md

@@ -0,0 +1,18 @@
+## Self-Referencing Type Hints
+
+Inside of a class, if you write a type hint that refers to the same class Python will raise an Error because the class is not known yet.   
+For example:
+```python
+class Money:
+    def __add__(self, other: Money) -> Money:  # ERROR: Money is unknown
+        """Return the sum of two money objects."""
+        ... (code omitted)
+```
+
+to fix this, add the following import:
+```python
+from __future__ import annotations
+
+class Money:
+    def __add__(self, other: Money) -> Money:  # OK!
+```