Build instructions

[gcb]

experience if missing C compiler dependencies is not very good.

After someone setups for elixir development, this is the experience:

==> bcrypt_elixir
could not compile dependency :bcrypt_elixir, "mix compile" failed. Errors may have been logged above.

There's no errors above. that is all the output. And this is a essential beginner step from the official guide.

I not that familiar with erl/rebar development but i can help if guided.