Meta: add stylelint with config

CONTRIBUTING.md

@@ -24,3 +24,18 @@ Examples:
 
 - `[TIDY] CI: Move common parts to own job`
 - `Style: Make navbar sticky`
+
+## Style guide
+We use stylelint to lint the (s)css, please run before you commit
+```shell
+(p)npm lint-fix
+```
+### Who to setup
+We would recommend using pnpm. [Install Guide](https://pnpm.io/installation)
+```shell
+pnpm i
+``` 
+You can also use npm if you want
+```shell
+npm i
+```