NODE_ENV log or readme.md update

[ChrisHurt]

https://github.com/mshick/add-pr-comment/blob/7c0890544fb33b0bdd2e59467fbacb62e028a096/src/main.ts#L151-L153

Since NODE_ENV is a reserved environment variable in this action, this should be mentioned in the readme or in a new log.

[mshick]

Excellent point. I think it might make more sense for me to use a more specific env var, to avoid any collisions. I'll get this sorted out soon. Thanks for suggesting!