add elixir implementation of dataloader (#129)

Author: zbarnes757

README.md

@@ -468,6 +468,8 @@ Looking to get started with a specific back-end? Try the [loaders in the example
   * [bs-dataloader](https://github.com/ulrikstrid/bs-dataloader)
 * Java
   * [java-dataloader](https://github.com/graphql-java/java-dataloader)
+* Elixir
+  * [dataloader](https://github.com/absinthe-graphql/dataloader)
 * Golang
   * [Dataloader](https://github.com/nicksrandall/dataloader)