split up yarn and npm install instructions for easier copying (#993)

Author: amantiwari1

docs/introduction/getting-started.md

@@ -44,7 +44,11 @@ Redux Toolkit is available as a package on NPM for use with a module bundler or
 ```bash
 # NPM
 npm install @reduxjs/toolkit
+```
+
+or
 
+```bash
 # Yarn
 yarn add @reduxjs/toolkit
 ```