Add Ecto section and getting started link to Elixir (EbookFoundation#2599)

Author: zwhitten

free-programming-books.md

@@ -35,6 +35,7 @@
 * [DTrace](#dtrace)
 * [Elasticsearch](#elasticsearch)
 * [Elixir](#elixir)
+  * [Ecto](#ecto)
   * [Phoenix](#phoenix)
 * [Emacs](#emacs)
 * [Embedded Systems](#embedded-systems)
@@ -1044,6 +1045,11 @@
 * [Learning Elixir](http://learningelixir.joekain.com) - Joseph Kain Blog (HTML)
 
 
+#### Ecto
+
+* [Ecto Getting Started Guide](https://hexdocs.pm/ecto/getting-started.html#content) (HTML)
+
+
 #### Phoenix
 
 * [Phoenix v1.3.0 Guide](https://hexdocs.pm/phoenix/overview.html) (HTML)