[Documentation] Add telegraf documentation

[aimtsou]

General Issues

This is mostly a documentation issue. I have seen there are tries to send metrics from telegraf to m3db with the influxdb output and without the usage of prometheus. As sections you could describe the following:

• Create documentation about how to use telegraf with m3db
• Provide a default configuration for the components needed.
• Cross check if metrics are being sent

These are the following issues which deal with telegraf:

1. [coordinator] influxdb write goes not support gzip #3412
2. [query] influxdb write improvements #3373
3. [coordinator] Influxdb importer endpoint (at /api/v1/influxdb/write) #2083

Please provide the following information along with a description of the issue that you're experiencing:

1. What service is experiencing the issue? (M3Coordinator, M3DB, M3Aggregator, etc)

I guess here M3coordinator is needed on the same node as telegraf

2. What is the configuration of the service? Please include any YAML files, as well as namespace / placement configuration (with any sensitive information anonymized if necessary).

Provide a default configuration for it

3. How are you using the service? For example, are you performing read/writes to the service via Prometheus, or are you using a custom script?

We want telegraf to send data to m3db