Document .env files support

[michalkvasnicak]

Spust already supports .env and .env.local files. Each of the files is optional. They has to be in a root directory of a project.

If user wants to use them they work as following:

1. file .env is loaded and parsed to variable (it is not assigned to process.env.* so it is not available in a configuration for example, see the last point)
2. file .env.local is loaded and parsed to a variable, override previous values for intersected keys from .env file.
3. all variables are available in your build as process.env.{name of variable} but they aren't available outside of the build. For example, you can't access them in spust.config.js. They are just interpolated using webpack.DefinePlugin()