Add some more notes on how to do HTML render

[PythonCHB]

Now that the HTML render assignment is now the last exercise (generally a good move) -- there's no chance, at least in the in-sync version to review it with he students after they've done it.

But there were a few "gotcha's" that showed up in their work this time, so it would be good to put some more notes in there. about it.

I need to review again their submissions, but one I rember is that a few folks put code like:

if self.tag == 'html':
    do_something_special

which really defeats the purpose of subclassing :-)

Also: it would be good to refactor that instructions (and tests) a bit to put a placeholder tag for the Element base class -- students get a bit confused about what it's for if it has an "html" tag, then why do you make an Html subclass?