02-holy-grail-mockup: Add checkboxes to the README

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

Author: gs-101

intermediate-html-css/advanced-grid/02-holy-grail-mockup/README.md

@@ -18,18 +18,18 @@ If you use the tools in the Advanced Grid Properties lesson you should be able t
 ![desired outcome stretched](./desired-outcome-stretched.png)
 
 ### Self Check
-- The container element has two columns
-- The container's second column is 4 times larger than the first column
-- The container element has a gap of 4px
-- The header element has two columns
-- The `ul` inside the menu element contains another grid
-- The `ul` inside the nav element contains another grid
-- The sidebar element has a gap of 50px
-- The text elements in the sidebar are centered with grid
-- The article element should set grid columns using `repeat` along with the `auto-fit` and `minmax` properties
-- The article columns should have a minimum value of 250px and a maximum of 1fr unit
-- The article element has a gap of 15px
-- The card elements inside the article container have a height of 200px
-- The header and footer span across both columns
-- The sidebar only spans across the first column
-- The nav and article elements only span across the second column
+- [ ] The container element has two columns
+- [ ] The container's second column is 4 times larger than the first column
+- [ ] The container element has a gap of 4px
+- [ ] The header element has two columns
+- [ ] The `ul` inside the menu element contains another grid
+- [ ] The `ul` inside the nav element contains another grid
+- [ ] The sidebar element has a gap of 50px
+- [ ] The text elements in the sidebar are centered with grid
+- [ ] The article element should set grid columns using `repeat` along with the `auto-fit` and `minmax` properties
+- [ ] The article columns should have a minimum value of 250px and a maximum of 1fr unit
+- [ ] The article element has a gap of 15px
+- [ ] The card elements inside the article container have a height of 200px
+- [ ] The header and footer span across both columns
+- [ ] The sidebar only spans across the first column
+- [ ] The nav and article elements only span across the second column