Add Setup/Contribution Instructions

[jwworth]

Hi! Thanks for building this library. It's awesome; we're using it over at: https://github.com/hashrocket/tilex

We're getting the following warnings at compile:

warning: Kernel.to_char_list/1 is deprecated, use Kernel.to_charlist/1
  lib/extwitter/oauth.ex:38

warning: Kernel.to_char_list/1 is deprecated, use Kernel.to_charlist/1
  lib/extwitter/oauth.ex:46

warning: String.to_char_list/1 is deprecated, use String.to_charlist/1
  lib/extwitter/proxy.ex:28

warning: String.to_char_list/1 is deprecated, use String.to_charlist/1
  lib/extwitter/proxy.ex:38

warning: String.to_char_list/1 is deprecated, use String.to_charlist/1
  lib/extwitter/proxy.ex:39

I'd love try fixing them, but I'm not sure how to set up the application and get the tests passing. Thank you!