docs: Add logo (#9574)

(cherry picked from commit 4928d5b)

README.md

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-# Telegraf [![Circle CI](https://circleci.com/gh/influxdata/telegraf.svg?style=svg)](https://circleci.com/gh/influxdata/telegraf) [![Docker pulls](https://img.shields.io/docker/pulls/library/telegraf.svg)](https://hub.docker.com/_/telegraf/) [![Total alerts](https://img.shields.io/lgtm/alerts/g/influxdata/telegraf.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/influxdata/telegraf/alerts/)
+
+# Telegraf
+
+![tiger](TelegrafTiger.png "tiger")
+
+[![Circle CI](https://circleci.com/gh/influxdata/telegraf.svg?style=svg)](https://circleci.com/gh/influxdata/telegraf) [![Docker pulls](https://img.shields.io/docker/pulls/library/telegraf.svg)](https://hub.docker.com/_/telegraf/) [![Total alerts](https://img.shields.io/lgtm/alerts/g/influxdata/telegraf.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/influxdata/telegraf/alerts/)
 [![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://www.influxdata.com/slack)
 
 Telegraf is an agent for collecting, processing, aggregating, and writing metrics.