Update to README describing how to create a composite profiles to avoid ...

A proposed update to the "usage" section of the README that briefly describes how keep env settings in profile.clj while not clobbering any profiles defined in project.clj.

I suffered the same confusion as expressed in the issue...

Creating a :dev profile in profiles.clj overrules the :dev profile in projects.clj (Or: I don't understand how this is supposed to work) #15