Mention selecting nodejs in "simple setup" instructions

[lennart]

since heroku now offers a myriad of different programming languages to run your app on, it would be great to mention selecting NodeJS as the Language in the second step of the Simple setup instructions, after following the link.

this was not obvious for someone trying to follow these instructions and since Install manually already calls it NodeJS Server environment (assuming you already know it is a programming language_, the rest of the installation documentation doesn't help here I think.

thank you