docs: Improve Usage Docs a bit around usage

I first thought I needed to run `mix compile.unused` which...
did nothing. So I think this will help folks.

I don't think the dependency and compiler should necessarily
be used on production as well so also added this.

Thanks for your work!

README.md

@@ -36,13 +36,20 @@ defmodule MySystem.MixProject do
       # In case of Phoenix projects you need to add it to the list
       # compilers: [:unused, :phoenix, :gettext] ++ Mix.compilers()
       # ...
+      #
+      # If you want to only run it in the dev environment you could do
+      # it by using `compilers: compilers(Mix.env()) ++ Mix.compilers()`
+      # instead and then returning the right compilers per environment.
     ]
   end
 
   # ...
 end
 ```
 
+Then you just need to run `mix compile` or `mix compile --force` as usual
+and unused hints will be added to the end of the output.
+
 ### Warning
 
 This isn't perfect solution and this will not find dynamic calls in form of: