Update readme

Author: carlpett

README.md

@@ -1,6 +1,14 @@
-## zookeeper_exporter [![CircleCI](https://circleci.com/gh/carlpett/zookeeper_exporter.svg?style=shield)](https://circleci.com/gh/carlpett/zookeeper_exporter)
+# zookeeper_exporter [![CircleCI](https://circleci.com/gh/carlpett/zookeeper_exporter.svg?style=shield)](https://circleci.com/gh/carlpett/zookeeper_exporter) [![DockerHub](https://img.shields.io/docker/build/carlpett/zookeeper_exporter.svg?style=shield)](https://hub.docker.com/r/carlpett/zookeeper_exporter/)
 
-A very simple prometheus exporter for zookeeper 3.4+. 
+A Prometheus exporter for Zookeeper 3.4+. It send the `mntr` command to a Zookeeper node and converts the output to Prometheus format. 
 
-### Limitations
+## Usage
+Download the [latest release](https://github.com/carlpett/zookeeper_exporter/releases), pull [the Docker image](https://hub.docker.com/r/carlpett/zookeeper_exporter/) or follow the instructions below for building the source.
+
+There is a `-help` flag for listing the available flags.
+
+## Building from source
+`go get -u github.com/carlpett/zookeeper_exporter` and then `make build`.
+
+## Limitations
 Due to the type of data exposed by Zookeeper's `mntr` command, it currently resets Zookeeper's internal statistics every time it is scraped. This makes it unsuitable for having multiple parallel scrapers.