01-css-methods: 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/01-css-methods/README.md

@@ -17,8 +17,8 @@ The properties you need to add to each element are:
 
 
 ### Self Check
-- Did you use all three methods of adding CSS to an HTML file?
-- Did you properly link the external CSS file in the HTML file?
-- Does the `div` element have CSS added via the external method?
-- Does the `p` element have CSS added via the internal method?
-- Does the `button` element have CSS added via the inline method?
+- [ ] Did you use all three methods of adding CSS to an HTML file?
+- [ ] Did you properly link the external CSS file in the HTML file?
+- [ ] Does the `div` element have CSS added via the external method?
+- [ ] Does the `p` element have CSS added via the internal method?
+- [ ] Does the `button` element have CSS added via the inline method?