Clarify that postgres contrib modules are required, not just postgres.

README.md

@@ -25,7 +25,7 @@ While we don’t provide docker files, other people have written very good ones.
 
 ### Dependencies
 
-* Postgresql version 9.6 or newer
+* Postgresql version 9.6 or newer, including the contrib modules
 * Elixir version 1.7 or newer. If your distribution only has an old version available, check [Elixir’s install page](https://elixir-lang.org/install.html) or use a tool like [asdf](https://github.com/asdf-vm/asdf).
 * Build-essential tools