Documentation could be more obvious about installation pitfall in :prod

[eriknaslund]

The install instructions do indeed mention that you should add the library using only: :dev, runtime: false.
However if you fail to add the dependency just like that you get an error, but only when trying to run your app in the :prod environment (because Mix isn't available).

Kernel pid terminated (application_controller) ("{application_start_failure,hex,{bad_return,{{'Elixir.Hex.Application',start,[normal,[
]]},{'EXIT',{undef,[{'Elixir.Mix.SCM',append,['Elixir.Hex.SCM'],[]},{'Elixir.Hex.Application',start,2,[{file,\"lib/hex/application.ex\
"},{line,9}]},{application_master,start_it_old,4,[{file,\"application_master.erl\"},{line,293}]}]}}}}}") 

I'd love it if the README.md could be more explicit about this issue, because it's one of those things that aren't immediately obvious (at least not to me).