[docs] How create an Addon and/or a Plugin?

[bittner]

Current Situation

There is an Addon called Douban.FM, which is whetting my appetite for writing an Addon myself, but there is not documentation how to do that, neither for addons nor for plugins.

Expected Situation

There should be concise but complete documentation about how to develop an addon or a plugin for PhoenixPlayer (with step-by-step instructions ideally) in the README.