Better documentation around collectd + InfluxDB and Telegraf

[jackzampolin]

Originally: https://github.com/influxdb/influxdb.com/issues/501

Please add to the docs:

• How to configure collectd to send data to InfluxDB
• How to configure collectd in the InfluxDB Managed Hosting environment
• Documentation about the native collectd plugin

[beckettsean]

closing in favor of #147