Adds documentation on environment variables.
Adds documentation on environment variables.

This would go towards solving #949. It's a page adapted from the
Rakudo one, which is linked, along with author attribution.

perl6/doc,jonathanstowe/doc,stmuk/doc,stmuk/doc,MasterDuke17/doc,tbrowder/doc,tbrowder/doc,tbrowder/doc,dmaestro/doc,jonathanstowe/doc,perl6/doc,MasterDuke17/doc,perl6/doc,dmaestro/doc,stmuk/doc,MasterDuke17/doc,MasterDuke17/doc,dmaestro/doc,stmuk/doc,dmaestro/doc,jonathanstowe/doc,perl6/doc,jonathanstowe/doc,dmaestro/doc,stmuk/doc,jonathanstowe/doc,perl6/doc,tbrowder/doc