add tip about nodenv permissions (#365)

Co-authored-by: Chiedo John <2156688+chiedo@users.noreply.github.com>

contributing/node-versions.md

@@ -19,10 +19,17 @@ When updating to a new Node.js version, consider the following files:
 
 If you're using macOS, run this command to get the latest:
 
-```
+```sh
 brew upgrade nodenv node-build
 ```
 
+If you see a warning like this one, run the suggested command:
+
+```sh
+# You should change the ownership of these directories to your user.
+sudo chown -R $(whoami) /usr/local/sbin
+```
+
 If you're using another operating system, or did not use Homebrew to install nodenv, see these [upgrade instructions](https://github.com/nodenv/nodenv#installation).
 
 To install Node.js 14 and make it your default version, run this command: