Better document recommended usage of the file input

[acchen97]

We should better communicate two things with the file input docs:

• Filebeat should be the recommended solution for log extraction.
• When absolutely required, the file input should only be used for file tailing.

Perhaps a banner statement similar to this would go a long way:

"For all log file extraction use cases, Filebeat is the strongly recommended solution coupled with Logstash or Elasticsearch for further processing and analytics. When required, the file input plugin for Logstash should only be used for the file tailing use case."

/cc @jsvd @tsg would appreciate your feedback as well.

[karenzone]

Follow-up discussions in Ingest channel (Wed, March 28). Broader discussions are underway on best approach/messaging. Hold off iomplementing this one for now.

[fbaligand]

@acchen97
With the new PR that is currently reviewed (#171 ), file input will have an option to indicate "tail" or "read" mode.