docs: add installation commands (#173)

Author: harish-sethuraman

README.md

@@ -19,10 +19,22 @@ Extracts source maps from existing source files (from their <code>sourceMappingU
 
 To begin, you'll need to install `source-map-loader`:
 
-```bash
+```console
 npm i -D source-map-loader
 ```
 
+or
+
+```console
+yarn add -D source-map-loader
+```
+
+or
+
+```console
+pnpm add -D source-map-loader
+```
+
 Then add the plugin to your `webpack` config. For example:
 
 **file.js**