Updating docs to indicate node pm2 also uses `username` environment variable if not escaped

[meditatingdragon]

Updating documentation informing users to escape userField: \\username if running Feathers with PM2, as it also just pulls the local environment variable username instead of the usernameField.

[daffl]

Thank you! Makes sense to mention in here for now - and another reason why this kind of environment variable substitution will be removed in the upcoming version.