01-flex-center: Add checkboxes to the README

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

Author: gs-101

foundations/flex/01-flex-center/README.md

@@ -7,6 +7,6 @@ All you need to do is center the red div inside the blue container.
 ![outcome](./desired-outcome.png)
 
 ### Self Check
-- Is the red div centered?
-- Did you _only_ use flexbox to center it?
+- [ ] Is the red div centered?
+- [ ] Did you _only_ use flexbox to center it?