Advertise DEBUG=1 in `rebar3 help`
Advertise DEBUG=1 in `rebar3 help`

Since we switched to DIAGNOSTIC=1 in the crashdump, the only way to make
DEBUG=1 discoverable (outside of docs) is in the help provider.

lrascao/rebar3,rebar/rebar3,rebar/rebar3,saleyn/rebar3,vladdu/rebar3,erlang/rebar3