Documentation should recommend one single way to structure plugin

[Tarrasch]

Basically I would prefer if antigen told the user how to setup their plugin so antigen will load it, rather than it mentioning the 100 different ways it supports. That is because as a plugin author, I just expect a one line explanation like: "Antigen will load any file in following the *.plugin.zsh format, it's behavior is undefined if multiple such file exists". I believe that the current docs is too long and slack.

Please also see issue #23, where we actually discuss which pattern should be the recommended one if any. Here we discuss if there should be a recommended pattern at all or not.