readme: Changed slack badge. (thanos-io#586)

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

Author: bwplotka

README.md

@@ -2,7 +2,7 @@
 
 [![CircleCI](https://circleci.com/gh/improbable-eng/thanos.svg?style=svg)](https://circleci.com/gh/improbable-eng/thanos)
 [![Go Report Card](https://goreportcard.com/badge/github.com/improbable-eng/thanos)](https://goreportcard.com/report/github.com/improbable-eng/thanos)
-[![Slack](docs/img/slack.png)](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc)
+[![Slack](https://img.shields.io/badge/join%20slack-%23thanos-brightgreen.svg)](https://join.slack.com/t/improbable-eng/shared_invite/enQtMzQ1ODcyMzQ5MjM4LWY5ZWZmNGM2ODc5MmViNmQ3ZTA3ZTY3NzQwOTBlMTkzZmIxZTIxODk0OWU3YjZhNWVlNDU3MDlkZGViZjhkMjc)
 
 ## Overview