[readme] Add quick intro to `nvm`

Allow newbies to get a quick understanding of how `nvm` works right from the start.

README.md

@@ -5,6 +5,7 @@
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 ## Table of Contents
 
+- [Intro](#intro)
 - [About](#about)
 - [Installing and Updating](#installing-and-updating)
   - [Install & Update Script](#install--update-script)
@@ -55,6 +56,29 @@
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
+## Intro
+
+`nvm` allows you to quickly install and use different versions of node via the command line.
+
+**Example:**
+```sh
+$ nvm use 16
+Now using node v16.9.1 (npm v7.21.1)
+$ node -v
+v16.9.1
+$ nvm use 14
+Now using node v14.18.0 (npm v6.14.15)
+$ node -v
+v14.18.0
+$ nvm install 12
+Now using node v12.22.6 (npm v6.14.5)
+$ node -v
+v12.22.6
+```
+
+Simple as that!
+
+
 ## About
 nvm is a version manager for [node.js](https://nodejs.org/en/), designed to be installed per-user, and invoked per-shell. `nvm` works on any POSIX-compliant shell (sh, dash, ksh, zsh, bash), in particular on these platforms: unix, macOS, and windows WSL.