05-descendant-combinator: 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/05-descendant-combinator/README.md

@@ -14,5 +14,5 @@ The properties you need to add are:
 
 
 ### Self Check
-- Do the elements that contain the text "This should be styled" have the correct styles applied?
-- Do the elements that contain the text "This should be unstyled" have no styles applied?
+- [ ] Do the elements that contain the text "This should be styled" have the correct styles applied?
+- [ ] Do the elements that contain the text "This should be unstyled" have no styles applied?