Added a list of known differences to the docs.

Rather than constantly responding to new bug reports for known 
differences between Python-Markdown and other implementations,
I've added this section to the docs to definitively state
our position and point people to workarounds. I'm sure some
people will still file reports, but at least I have something
to point them to.

If there are any I missed, I can always add them latter 
when they come up.