Document common error when node is not sufficiently recent

Author: badeball

docs/faq.md

@@ -3,3 +3,7 @@
 ### `--env` / `tags` isn't picked up
 
 This might be because you're trying to specify `-e / --env` multiple times, but [multiple values should be comma-separated](https://docs.cypress.io/guides/guides/command-line#cypress-run-env-lt-env-gt).
+
+### I get `fs_1.promises.rm is not a function`
+
+Upgrade your node version to at least [v14.14.0](https://nodejs.org/api/fs.html#fspromisesrmpath-options).