Add a guide to set up password authentication for in/out_forward

[fujimotos]

This patch adds a step-by-step instruction to enable the challenge-response authentication mechanism, which was introduced in Fluentd v0.14.5.

For details, see: https://github.com/fluent/fluentd/pull/1136

Since out_forward has not supported TLS mutual authentication yet, this is the only built-in mechanism to prevent rogue clients from connecting. So this guide should be still somewhat useful.

Signed-off-by: Fujimoto Seiji fujimoto@clear-code.com