Add Elixir to client libraries (#71)

Author: onvlt

content/4.developer/2.client-libraries.md

@@ -15,7 +15,7 @@ There are a number of client libraries available to help send e-mail using the P
 * [.Net](https://github.com/KingdomFirst/PostalServer-DotNet-Framework)
 * [.NET Core](https://github.com/mDev86/PostalApiClient)
 * [Go](https://github.com/Pacerino/postal-go)
-
+* [Elixir (via Swoosh)](https://hexdocs.pm/swoosh/Swoosh.Adapters.Postal.html)
 
 All of these libraries will make use of the API rather than using any SMTP protocol - this is considered to be best approach for delivering your messages.