add docs about using nvm to readme and contributing.md (swiftlang#529)

Author: QuietMisdreavus

CONTRIBUTING.md

@@ -51,7 +51,9 @@ more details.
 ### Run Steps
 
 > Note: requires [Node.js](https://nodejs.org/en/download/) v14
-> and [npm](https://www.npmjs.com/package/npm) v6.14
+> and [npm](https://www.npmjs.com/package/npm) v6.14. An easy way to set these up is to install
+> [nvm](https://github.com/nvm-sh/nvm) and run `nvm install` from within the Swift-DocC-Render
+> repository. To use these versions as the default, add `--default` to the installation command.
 
 1. Checkout this repository using:
 

README.md

@@ -8,7 +8,9 @@ SPAs are web apps that render dynamically at runtime entirely in the browser, us
 ## Getting Started
 
 > Note: requires [Node.js](https://nodejs.org/en/download/) v14
-> and [npm](https://www.npmjs.com/package/npm) v6.14
+> and [npm](https://www.npmjs.com/package/npm) v6.14. An easy way to set these up is to install
+> [nvm](https://github.com/nvm-sh/nvm) and run `nvm install` from within the Swift-DocC-Render
+> repository. To use these versions as the default, add `--default` to the installation command.
 
 1. **Download this repository and go to its folder**