This adds changes to the rebar3 expression that patch rebar3 to force it
to be hermetic. Now, by default, rebar3 literally can't download
anything. A 'rebar3-open' expression was added for those folks whe want
the normal rebar3.
This commit adds some very minimial documentation to the Nix
manual. Hopefully, its enough to get someone started and serve as a
first footstep for future documentation writers