InstallFest: Add classes cheatsheet

Author: hroncok

runs/2018/installfest/info.yml

@@ -42,6 +42,9 @@ plan:
   - title: Slovníkový tahák
     url: https://pyvec.github.io/cheatsheets/dicts/dicts-cs.pdf
     type: cheatsheet
+  - title: Tahák na třídy
+    url: https://pyvec.github.io/cheatsheets/classes/classes.pdf
+    type: cheatsheet
   - title: Tahák na Pyglet
     url: https://pyvec.github.io/cheatsheets/pyglet/pyglet-basics-cs.pdf
     type: cheatsheet