Framework: Pin nvmrc to specific current LTS (#22236)

* Framework: Pin nvmrc to specific current LTS

* Docs: Update Getting Started to note nvmrc pinning

Author: aduth

.nvmrc

@@ -1 +1 @@
-lts/*
+12

docs/contributors/getting-started.md

@@ -1,8 +1,11 @@
 # Getting Started
 
-Gutenberg is a Node.js-based project, built primarily in JavaScript.
 
-The first step is to install the [latest active LTS release](https://github.com/nodejs/Release#release-schedule) of Node, along with the latest version of [NPM](https://www.npmjs.com/). The easiest way (on macOS, Linux, or Windows 10 with the Linux Subsystem) is by installing and running [nvm]. Once `nvm` is installed, you can install the correct version of Node by running `nvm install --latest-npm` in the Gutenberg directory.
+Gutenberg is built using the [latest active LTS release](https://github.com/nodejs/Release#release-schedule) of [Node.js](https://nodejs.org/en/), along with the latest version of [NPM](http://npmjs.com/).
+
+The easiest way to install and manage node and NPM (on macOS, Linux, or Windows 10 with the Linux Subsystem) is by using [nvm](https://github.com/creationix/nvm). Once nvm is installed, you can install the correct version of Node by running nvm install --latest-npm in the Gutenberg directory.
+
+>**Note:** If you find yourself needing to build older versions of Gutenberg, nvm makes that process easier too. Because Gutenberg's `.nvmrc` file is regularly updated to the current available LTS release, running `nvm install` on an older branch will install whichever LTS version was active at that time.
 
 Once you have Node installed, run these scripts from within your local Gutenberg repository:
 
@@ -64,9 +67,6 @@ After building the cloned gutenberg directory contains the complete plugin, you
 
 Another way to upload after building is to run `npm run package-plugin` to create a plugin zip file — this requires `bash` and `php` to run. The script creates `gutenberg.zip` that you can use to install Gutenberg through the WordPress admin.
 
-[npm]: https://www.npmjs.com/
-[nvm]: https://github.com/creationix/nvm
-
 ## Storybook
 
 > Storybook is an open source tool for developing UI components in isolation for React, React Native and more. It makes building stunning UIs organized and efficient.