Add documentation details for environment variables usage

[M0Rf30]

I have a Java project and I want to integrate pacur to it's maven build. Problem is, I can't build packages using some build specific parameters such as the project version without manually editing or using sed on the PKGBUILD file.

With this PR, now I can do something like:

pkgver=${VERSION}

and supply the VERSION environment variable to yap in order to compile a package with the given version, like:

docker run --rm -t -v $(pwd):/yap -e VERSION=1.0.1 yap/ubuntu-bionic

or

VERSION=1.0.2 yap build ubuntu example

Originally posted by @apontini in #3 (comment)