chore: update readme to include bun install

Author: Electroid

README.md

@@ -60,6 +60,12 @@ or
 pnpm add -D postcss-loader postcss
 ```
 
+or
+
+```console
+bun add -D postcss-loader postcss
+```
+
 Then add the plugin to your `webpack` config. For example:
 
 > In the following configuration the plugin [`postcss-preset-env`](https://github.com/csstools/postcss-preset-env) is used, which is not installed by default.