Update README.md instructing first-timers to add env vars

[devxpy]

This would really help people who are new to this plugin.

Just today I was frustrated why this didn't work out of the box and realised it was just a matter of setting some env vars.

[ovv]

I believe those env vars should be set automatically by the plugin. You might have hit the same bug as #14.

[devxpy]

It's a good replacement until everything is figured out.

I can't emphasise on how much headache stuff like this causes. A simple "NOTE" goes a long way...