06-flex-layout: Add checkboxes to the README

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

Author: gs-101

foundations/flex/06-flex-layout/README.md

@@ -12,11 +12,11 @@ Flexbox is useful for laying out entire pages as well as the smaller components
 
 ### Self Check
 
-- The header is at the top of the page, the footer is at the bottom, and they stay in place if you resize your screen.
-- The header and footer have padding.
-- The links in the header and footer are pushed to either side.
-- There is space between the links in the header and footer.
-- The footer has a light gray background (`#eeeeee`).
-- The logo, input and buttons are centered in the screen.
-- The buttons have an appropriate amount of padding.
-- There is space between the logo, input and buttons.
+- [ ] The header is at the top of the page, the footer is at the bottom, and they stay in place if you resize your screen.
+- [ ] The header and footer have padding.
+- [ ] The links in the header and footer are pushed to either side.
+- [ ] There is space between the links in the header and footer.
+- [ ] The footer has a light gray background (`#eeeeee`).
+- [ ] The logo, input and buttons are centered in the screen.
+- [ ] The buttons have an appropriate amount of padding.
+- [ ] There is space between the logo, input and buttons.