add ecto usage to readme

[simonmcconnell]

Clarifies how to use it with Ecto. The lack of Ecto instructions led to issue #155.

[mjaric]

Maintainer of ecto_sql adapter may change they way how parameters are configured in app for adapter. Adapter is responsible to respect driver start_link parameters... Having said that, the right place is include such details in ecto_sql and not here.

Anyhow, I don't think this is lacking in ecto docs.

Here is how repo is configured (like in your example)
Here is list of built-in adapters in repo
Here we already have list of connection parameters for TDS addapter that you can put in config YourRepoModule, ....

What ecto is missing is probably some "Quick Start"s for each builtin adapter.