change to note between 2.0 and 2.1

[Tavis Rudd]

Hi Andrew,
the re module handles negative 
look-behinds differently in 2.1 and 2.0.  In 2.1 a 
negative look-behind at the very beginning of a string 
will match if the rest of the following text matches. In 2.0
this doesn't match.  Would you mind adding a note about 
this on the "What's New in Python 2.1" page?

example
=========================================
Try to match all $, but not \$, in the following string.
"$ $ $ \$"

Using re.compile(r"(?<!\\)\$"), Python 2.1 finds 3, 
Python 2.0 finds 2.

Using re.compile(r"(?:(?<=\A)|(?<!\\))\$"), both versions 
find 3.
=========================================
Cheers,
Tavis