Document the fomat of the object returned by getEnvironment()

[tunniclm]

The README.md should describe (but currently doesn't) the keys / values available in the object returned by getEnvironment().

[mattcolegate]

Wouldn't this be difference depending on the host system? Also don't we pick up user-defined variables that are impossible to know ahead of time? We could probably document the steps of what we capture rather than the key/value pairs e.g. "the output of env". Do you think that is sufficient?

[sjanuary]

There's a set we'll get every time provided by nodeenvplugin.cpp which could be documented, plus a general comment about where the rest come from.