add central badge

README.md

@@ -1,8 +1,8 @@
 # doobie
 
 [![Travis CI](https://travis-ci.org/tpolecat/doobie.svg?branch=master)](https://travis-ci.org/tpolecat/doobie)
-
 [![Join the chat at https://gitter.im/tpolecat/doobie](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/tpolecat/doobie?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Maven Central](https://img.shields.io/maven-central/v/org.tpolecat/doobie-core_2.11.svg)](https://maven-badges.herokuapp.com/maven-central/org.tpolecat/doobie-core_2.11)
 
 **doobie** is a pure functional JDBC layer for Scala. It is not an ORM, nor is it a relational algebra; it just provides a principled way to construct programs (and higher-level libraries) that use JDBC. **doobie** introduces very few new abstractions; if you are familiar with basic `scalaz` typeclasses like `Functor` and `Monad` you should have no trouble here.