Clarify branches master and develop

[lucafavatella]

It looks like releases are taken from branch develop - see #24 (comment) - but the default branch of the repo is master: do I understand correctly that the branches shall be reviewed following practice in https://github.com/erlang-lager/lager i.e.:

1. Delete master branch;
2. Rename develop branch as master
   ?