docs: Add known issue

Author: vain0x

README.md

@@ -10,6 +10,10 @@
 > - Final upstream commit: `2020-12-19 20:16:02 -0300 f6bdfa0`
 > - First fork commit: `2022-09-05 00:08:50 +0900 d3672ba`
 
+### Known issues of the Fork
+
+- CSS that contains `@apply` doesn't get parsed correctly due to the parser change.
+
 # IntelliSense for CSS class names in HTML
 
 A Visual Studio Code extension that provides CSS class name completion for the HTML `class` attribute based on the definitions found in your workspace or external files referenced through the `link` element.