Create a solution / section for 3th party plugins for wp car manager.

[pgroot91]

Would be nice to integrate a section were 3th party plugins are showcased in the extension section. This way other plugins also get exposure and so are it's users able to find more extensions for wp-car-manager.

[pgroot91]

Made a start with updating the readme #211 so other users can find easely 3th party plugins that they can use with wp-car-manager.