Document default environment variables

[dgilleland]

As noted in the forum post "List of default environment variables", it would be helpful to have these environment variables listed in the documentation.

I would be happy to do a pull-request solution for this.

@knolleary - Is there any particular page you would like to see this documented? I'm thinking the docs/user-guide/environment-variables.md file.

[knolleary]

That is probably the best place at the moment.

[dgilleland]

Ok. I'll aim to have a pull request ready by Monday.