Add basic library usage to documentation
Add `Customising` section to documentation
Add ability for subclasses to be found in current directory
Update `README.rst`, remove usage and add Read the Docs link
Update `Usage` documentation to use Sphinx roles
Use Sphinx role in documentation
Improve `Customising` documentation

Add more information about the classes that make up Stackoverflow
Watcher and add some examples of subclassing.
Add a page to the documentation about contributing