Add `free` to list description

There has been some recent interest in adding paid services to this list, so adding `free` in the project's description to clarify the mission of the project.

README.md

 # Public APIs [![Build Status](https://api.travis-ci.org/toddmotto/public-apis.svg)](https://travis-ci.org/toddmotto/public-apis)
 
-A collective list of JSON APIs for use in web development.
+A collective list of free JSON APIs for use in web development.
 
 For information on contributing to this project, please see the [contributing guide](CONTRIBUTING.md).