Extension Writing doc update

[Michael P. Reilly]

I've updated the document I created a couple of weeks ago describing
how to write your own Python type objects in C.

I've added sections describing:
* How to write numeric callback methods (__add__, __invert__, etc.)
* How to write sequence callback methods (__len__, __add__, __getitem__)
* How to write mapping callback methods (__len__, __getitem__)
* How to add doc strings to your Python/C types
* A (for now) terse explaination on how to make your own coerce
function.

As before, the verbage isn't fleshed out.  I wanted to get the content
out there, more than the long winded descriptions.

The USL is http://starship.python.net/~arcege/extwriting/pyext.html. My
email address is arcege at shore.net.

  -Arcege