Update the developer guide on output plugins

[fujimotos]

Problem

As pointed out by the following comment, the article "Writing Output Plugins"
is still under development right now.

fluent/fluentd#1941 (comment)

We need to update this article so that developers can rely on (since there
is no other document on how to write output plugins).

Big Picture

The bigger picture here is that this is one of the last two articles
marked as This is simply copied from v0.12 (Note: the other one
is in the process of being fixed in #540). So we reach a good point
when we've done this article fixed.