07-flex-layout-2: Add checkboxes to the README

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

Author: gs-101

foundations/flex/07-flex-layout-2/README.md

@@ -23,13 +23,13 @@ On a smaller screen it will look like this:
 Note: The emojis may instead show up as one or several text symbols (e.g. ☆♂) if you don't have an emoji-based font family installed on your operating system. This does not affect the exercise and can be ignored.
 
 ### Self Check
-- The header text is size 32px and weight 900.
-- The header text is vertically centered and 16px from the edge of the screen.
-- The footer is pushed to the bottom of the screen (the footer may go _below_ the bottom of the screen if the content of the 'cards' section overflows and/or if your screen is shorter).
-- The footer text is centered horizontally and vertically.
-- The sidebar and cards take up all available space above the footer.
-- The sidebar is 300px wide (and it doesn't shrink).
-- The sidebar links are size 24px, are white, and do not have the underline text decoration.
-- The sidebar has 16px padding.
-- There is 48px padding around the 'cards' section.
-- The cards are arranged horizontally, but wrap to multiple lines when they run out of room on the page.
+- [ ] The header text is size 32px and weight 900.
+- [ ] The header text is vertically centered and 16px from the edge of the screen.
+- [ ] The footer is pushed to the bottom of the screen (the footer may go _below_ the bottom of the screen if the content of the 'cards' section overflows and/or if your screen is shorter).
+- [ ] The footer text is centered horizontally and vertically.
+- [ ] The sidebar and cards take up all available space above the footer.
+- [ ] The sidebar is 300px wide (and it doesn't shrink).
+- [ ] The sidebar links are size 24px, are white, and do not have the underline text decoration.
+- [ ] The sidebar has 16px padding.
+- [ ] There is 48px padding around the 'cards' section.
+- [ ] The cards are arranged horizontally, but wrap to multiple lines when they run out of room on the page.