doc: add recommendations for first timers

PR-URL: https://github.com/nodejs/node/pull/16350


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>