docs: add note on postcss 7 support

Ffloriel · Ffloriel · commit 24571594d0a2 · 2021-01-17T21:07:29.000Z
postcss 7 not supported on version 4.0 and above
diff --git a/docs/plugins/postcss.md b/docs/plugins/postcss.md
@@ -24,6 +24,11 @@ meta:
 
 # PostCSS
 
+::: warning
+If you are using PostCSS 7, install @fullhuman/postcss-purgecss 3.0.0: `npm i -D @fullhuman/postcss-purgecss@3.0.0`.
+From version 4.0, it is compatible with PostCSS >=8 only.
+:::
+
 ## Installation
 
 ```
