Build: Document how to use environment variables in Build plugins

[ehmicky]

We should document (in README) how to use environment variables in Build plugins.

The answer is: with process.env like any regular Node.js code.

A user today was wondering whether he should use netlifyConfig.build.environment instead.

We might want to be even more generic, saying that the environment should be as similar as possible to a regular Node.js environment, which includes filesystem, etc.

@verythorough @klavavej

[erquhart]

We can also handle this one if the docs team is more focused on docs.netlify.com content. Not certain if that's the case, I'll let them respond on that.

[verythorough]

If you'd like to make a pass adding this information to the current README, that'd be great. I can revise as needed when we're ready to move the plugin-building docs into docs.netlify.com.

[ehmicky]

Sounds good! Done at #1104.