02-class-id-selectors: Add checkboxes to the README

Add checkboxes to the README to make the progress tracking a better experience.

Author: gs-101

foundations/intro-to-css/02-class-id-selectors/README.md

@@ -20,6 +20,6 @@ Quick tip: in VS Code, you can change which format colors are displayed in RGB,
 
 
 ### Self Check
-- Do the odd numbered `p` elements share a class?
-- Do the even numbered `div` elements have unique IDs?
-- Does the Number 3 element have multiple classes?
+- [ ] Do the odd numbered `p` elements share a class?
+- [ ] Do the even numbered `div` elements have unique IDs?
+- [ ] Does the Number 3 element have multiple classes?