Update README with information about tests.

README.markdown

@@ -30,6 +30,9 @@ Enabling a new plugin or theme is as easy as it should be. Open your fish config
 As an example, to enable rails and git plugins add this line `set fish_plugins git rails`
 to your configuration file.
 
+## Tests
+Oh-My-Fish comes fully tested via `fish-spec`, a plugin bundled with the framework. To learn how to use `fish-spec` in your own projects, check out the [usage guide](plugins/fish-spec/README.markdown).
+
 ## Customization
 
 If you have many functions which go well together, you can create custom plugin in the `custom/plugins/PLUGIN_NAME`