Add worker section description

[cosmo0920]

This is an advanced usage for Fluentd.
Like fluent/fluentd#2145, users want to use multi workers but there is a few pitfall.
This pitfall is originated from multi worker unsupported plugin implementation.

It can avoid with <worker > section but I cannot find <worker > section usage documentation.

[fujimotos]

OK. I'll write some article on it next week.

[repeatedly]

I have just added multi process workers article.
If you have an idea for document improvement, please send a patch :)

[fujimotos]

That's awesome. Thank you!