Pin NPM version using Volta

Author: bertdeblock

README.md

@@ -38,7 +38,7 @@ If you run `ember serve`, the Markdown files are turned into HTML to create an a
 
 ## Local development
 
-**NOTE:** This project uses [Volta](https://volta.sh/) to ensure the correct Node.js version is used during local development.
+**NOTE:** This project uses [Volta](https://volta.sh/) to ensure the correct Node.js and NPM version is used during local development.
 
 To run the Ember Guides app locally, type these commands into your terminal.
 

package.json

@@ -110,6 +110,7 @@
     "edition": "octane"
   },
   "volta": {
-    "node": "12.19.1"
+    "node": "12.19.1",
+    "npm": "7.20.6"
   }
 }