Update README, add badges.

Signed-off-by: Alexey Palazhchenko <alexey.palazhchenko@gmail.com>

README.md

-# TSDB
+# TSDB [![Build Status](https://travis-ci.org/prometheus/tsdb.svg?branch=master)](https://travis-ci.org/prometheus/tsdb)
 
-This repository contains the new Prometheus storage layer that will be used in its 2.0 release.
+[![GoDoc](https://godoc.org/github.com/prometheus/tsdb?status.svg)](https://godoc.org/github.com/prometheus/tsdb)
+[![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/tsdb)](https://goreportcard.com/report/github.com/prometheus/tsdb)
+
+This repository contains the Prometheus storage layer that is used in its 2.x releases.
 
 A writeup of its design can be found [here](https://fabxc.org/blog/2017-04-10-writing-a-tsdb/).