Document switching node version using nvm so tests will work. (#8117)

* Document switching node version using nvm so tests will work.

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -16,6 +16,13 @@ The easiest way to get started (on MacOS, Linux, or Windows 10 with the Linux Su
 
 For other version of Windows, or if you prefer to set things up manually, be sure to have <a href="https://nodejs.org/en/">Node.js installed first</a>. You should be running a Node version matching the [current active LTS release](https://github.com/nodejs/Release#release-schedule) or newer for this plugin to work correctly. You can check your Node.js version by typing `node -v` in the Terminal prompt.
 
+If you have an incompatible version of Node in your development environment, you can use [nvm](https://github.com/creationix/nvm) to change node versions on the command line:
+
+```
+npx nvm install
+npx nvm use
+```
+
 You should also have the latest release of <a href="https://npmjs.org">npm installed</a>, npm is a separate project from Node.js and is updated frequently. If you've just installed Node.js which includes a version of npm within the installation you most likely will need to also update your npm install. To update npm, type this into your terminal: `npm install npm@latest -g`
 
 To test the plugin, or to contribute to it, you can clone this repository and build the plugin files using Node. How you do that depends on whether you're developing locally or uploading the plugin to a remote host.