No real documentation on why and how to use the plugin

[alexamici]

Minimal documentation needed:

• [x] Intro in the README.
• [x] Add some non trivial example - (strtobool) - moved to #13
• [x] Hosted documentation - http://pytest-wish.rtfd.org

Web presence:

• [x] nodev blog - http://blog.nodev.io

Review user visible documentation:

• [x] Command line help strings.
• [x] Error messages.