docs: add yarn and pnpm installation commands (#564)

Author: ersachin3112

README.md

@@ -49,6 +49,18 @@ To begin, you'll need to install `postcss-loader` and `postcss`:
 npm install --save-dev postcss-loader postcss
 ```
 
+or
+
+```console
+yarn add -D postcss-loader postcss
+```
+
+or
+
+```console
+pnpm 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.