[readme] add nvm intro

Closes #2149.

Author: flip111

Diff for: README.md

@@ -5,6 +5,7 @@
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 ## Table of Contents
 
+- [About](#about)
 - [Installing and Updating](#installing-and-updating)
   - [Install & Update Script](#install--update-script)
     - [Additional Notes](#additional-notes)
@@ -44,6 +45,9 @@
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
+## 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.
+
 <a id="installation-and-update"></a>
 <a id="install-script"></a>
 ## Installing and Updating