Add an environment variables section to User Guide

[GAumala]

Added new section with 5 environment variables that can be used with npm start:

• BROWSER
• HTTPS
• HOST
• NODE_PATH
• PORT

I placed this section right before troubleshooting because I'm not sure were it should go. This documents all the environment variables that I am aware of as well as the new BROWSER variable. I don't have any experience writing user guides so, any criticism is welcome, I'll try to make any changes you request.

[lucasavila00]

Hey @GAumala . That's awesome! 👍
But I guess this project is made for beginners too and the whole concept of Env vars may be something new for them. When I started programming I'd see people saying "This is simple. Just set the env var to foo and you're done :)" and I would be like "WAT??????????"
Past me would have read your tutorial and would have no idea what are you talking about. My point is that only mentioning env vars is bad for beginners.

But...
What about using dotenv and in the docs just tell people to change the .env file?
Experienced users would know what you're talking about and beginners won't be scared to change a line in a file. What do you think?
Thanks!

[GAumala]

Yeah, I definitely was thinking more about the advanced users, I pictured this to be something like an appendix. I guess I could add some examples with .env files, although that could make the section longer.

[GAumala]

There's a nice table explaining environment variables, so there's no need for this anymore.

[gaearon]

Oh, I’m so sorry we missed this 😢 . I’m sure @Timer didn’t see your PR when he was pushing 0.9.0 and adding that table. The PR list is in a bit of a messy state right now, and some PRs got lost. I’m going through them right now. Again, apologies.