Readme improvements

Note: doctests caught some errors in the code examples::

```
$ python -m doctest README.rst                                                                                                                                       **********************************************************************
File "README.rst", line 63, in README.rst
Failed example:
    -inf < inf
Expected:
    True
    >> inf == inf
    True
Got:
    True
**********************************************************************
File "README.rst", line 89, in README.rst
Failed example:
    5 / inf
Expected:
    0.0
Got:
    0
**********************************************************************
File "README.rst", line 91, in README.rst
Failed example:
    3 / -inf
Expected:
    -0.0
Got:
    0
**********************************************************************
File "README.rst", line 140, in README.rst
Failed example:
    float(inf)
Expected:
    float('inf')
Got:
    inf
**********************************************************************
File "README.rst", line 142, in README.rst
Failed example:
    float(-inf)
Expected:
    float('-inf')
Got:
    -inf
**********************************************************************
1 items had failures:
   5 of  25 in README.rst
***Test Failed*** 5 failures.
```