Add deps.get to installation instructions

When starting from a fresh copy of the source tree, it's necessary
to run the mix `deps.get` task.

Author: jwarlander

README.md

@@ -19,7 +19,7 @@ Update config/config.exs, then generate an executable
 ```
 mv config/config.exs.sample config/config.exs
 vim config/config.exs
-mix escript.build
+mix do deps.get, escript.build
 ```
 
 Run the executable