Clearly document our environment variables

[xzyfer]

Inspired by #1106 (and others I can't remember atm).

We have a handful of really useful environment variables, with subtly different behaviour. These are currently poorly, or undocumented which leads to people not knowing they exist, or how they work.

We need to document theses

Off the top of my head we ENV variables for installing/downloading the binaries, as well compiling the binary in dev mode. Below is some, but not all, of the ENV variables that directly affect node-sass.

Variable name	value
SASS_BINARY_NAME	path
SASS_BINARY_SITE	URL
SASS_BINARY_PATH	path
LIBSASS_EXT	auto,yes,no
LIBSASS_CFLAGS	compiler directives
LIBSASS_LDFLAGS	linker directives

[am11]

👍
Incidentally, should we add two more columns to list down their package.json cofig and CLI argument counterparts (where applicable)?