document the lack of automatic rebuilding after env var changes

[oconnor663]

Tracking issue: #230

[alexcrichton]

Thanks for this! I wonder though, if we're going to go to the effort to document this, would you be up for perhaps improving the current state of things? While I don't think we can emit rerun-if-* directives by default, it seems reasonable to have an option to opt-in to rerun-if directives perhaps? That way cc could print out all the env vars it reads, users could print out source files, and the note here in the README could mention the opt-in to use for watching changed env vars?

[JohnTitor]

Closing in favor of #701, thanks for the PR anyway!