Add troubleshooting documentation for Node 16 updates on M1 machines

If a user has installed govuk-frontend locally on an M1 machine using Node 14 or lower, `node-sass` will cause errors when running `npm install` or `npm start`.

This can be solved by following the steps in this commit.

This will only affect devs or contributors on Apple silicon who are updating from an older install.

docs/contributing/running-locally.md

@@ -29,6 +29,17 @@ We use [npm](https://docs.npmjs.com/getting-started/what-is-npm) to manage the d
 npm install
 ```
 
+### Fixing Node 16+ errors
+If you've previously installed `govuk-frontend` locally using Node v14 or earlier, you may see `node-sass`-related errors when updating to Node v16.
+
+To get rid of these errors, delete the `node_modules` folder, then run:
+
+```
+nvm use
+npm uninstall node-sass -g && npm cache clean -force && npm install node-sass
+npm install
+```
+
 ## 5. Start a local server
 This will build sources, serve pages and watch for changes.
 ```