Update README.md

Fix markdown and add link to more information on auto margins

Author: simonsmith

README.md

@@ -77,11 +77,13 @@ SUIT CSS flexbox utilities
 
 `X` can be any of the following numbers: `1`, `2`, `3`, `4`, `5`.
 
-** Aligning with `auto` margins **
+**Aligning with `auto` margins**
 
 * `u-flexExpandLeft` - Expand left margin to fill remaining space
 * `u-flexExpandRight` - Expand right margin to fill remaining space
 
+[Read more](https://medium.com/@samserif/flexbox-s-best-kept-secret-bd3d892826b6) about how this works.
+
 ### Plugins
 
 All flex utilities can be limited to specific Media Query breakpoints.