Add classes cheatsheet to classes lesson

[hroncok]

Done from browser, untested.

[hroncok]

BTW this is awesome (not applicable here):

[encukou]

There's something vague about the classes cheatsheet that worries me, but I can't seem to put it into words properly. Can we discuss in person (so I can either find the right words to document it for everyone, or see that I shouldn't be worried)?

[encukou]

We finally met about this!
The cheatsheet needs a few improvements:

• The arrow to snez should go to Prasatko
• Typová kontrola – change variable to prasatko, remove from the loop
• Make the explanations in the right column not depend on each other (where possible)
• Kotatko.vysun_drapky instead of zamnoukej; remove zakvic
• The Polymorphism section can be removed