03-grouping-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/03-grouping-selectors/README.md

@@ -14,6 +14,6 @@ This will help you further practice adding classes and using class selectors, so
 
 ### Self Check
 
-- Does each element have a unique class name?
-- Did you use the grouping selector for styles that both elements share?
-- Did you make separate rules for the styles unique to each element?
+- [ ] Does each element have a unique class name?
+- [ ] Did you use the grouping selector for styles that both elements share?
+- [ ] Did you make separate rules for the styles unique to each element?