Update css-in-javascript/README.md

Added the Link for the acronym and also expanding it to make it more understandable and accessible to the reader

Co-authored-by: Jordan Harband <[email protected]>

css-in-javascript/README.md

@@ -34,7 +34,7 @@
 
   - Use an underscore for modifiers to other styles.
 
-    > Why? Similar to BEM (Block Element Modifier), this naming convention makes it clear that the styles are intended to modify the element preceded by the underscore. Underscores do not need to be quoted, so they are preferred over other characters, such as dashes.
+    > Why? Similar to [BEM](https://getbem.com/introduction/), this naming convention makes it clear that the styles are intended to modify the element preceded by the underscore. Underscores do not need to be quoted, so they are preferred over other characters, such as dashes.
 
     ```js
     // bad