Expanded readme (python#1494)

Expanded readme slightly by adding links to PEP 484 and proper markup for the code example

README.md

@@ -9,23 +9,23 @@ What is mypy?
 
 Mypy is an optional static type checker for Python.  You can add type
 hints to your Python programs using the standard for type
-annotations introduced in Python 3.5 (PEP 484), and use mypy to
+annotations introduced in Python 3.5 ([PEP 484](https://www.python.org/dev/peps/pep-0484/)), and use mypy to
 type check them statically. Find bugs in your programs without even
 running them!
 
 The type annotation standard has also been backported to earlier
 Python 3.x versions.  Mypy supports Python 3.2 and later.
 
 For Python 2.7, you can add annotations as comments (this is also
-specified in PEP 484).
+specified in [PEP 484](https://www.python.org/dev/peps/pep-0484/)).
 
 You can mix dynamic and static typing in your programs. You can always
 fall back to dynamic typing when static typing is not convenient, such
 as for legacy code.
 
 Here is a small example to whet your appetite:
 
-```
+```python
 from typing import Iterator
 
 def fib(n: int) -> Iterator[int]: