docs: add yarn and pnpm installation commands (#1417)

Author: ersachin3112

README.md

@@ -29,6 +29,18 @@ To begin, you'll need to install `css-loader`:
 npm install --save-dev css-loader
 ```
 
+or
+
+```console
+yarn add -D css-loader
+```
+
+or
+
+```console
+pnpm add -D css-loader
+```
+
 Then add the plugin to your `webpack` config. For example:
 
 **file.js**