02-flex-header: Add checkboxes to the README

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

Author: gs-101

foundations/flex/02-flex-header/README.md

@@ -11,10 +11,10 @@ wide:
 ![wide](./desired-outcome-wide.png)
 
 ### Self Check
-- There is space between all items and the edge of the header (specific px amount doesn't matter here).
-- Logo is centered vertically and horizontally.
-- list-items are horizontal, and are centered vertically inside the header.
-- left-links and right-links are pushed all the way to the left and right, and stay at the edge of the header when the page is resized.
-- Your solution does not use floats, inline-block, or absolute positioning.
+- [ ] There is space between all items and the edge of the header (specific px amount doesn't matter here).
+- [ ] Logo is centered vertically and horizontally.
+- [ ] list-items are horizontal, and are centered vertically inside the header.
+- [ ] left-links and right-links are pushed all the way to the left and right, and stay at the edge of the header when the page is resized.
+- [ ] Your solution does not use floats, inline-block, or absolute positioning.
 
-- Note: For this exercise, it's completely acceptable to not match the font-family.
+Note: For this exercise, it's completely acceptable to not match the font-family.