update readme version, add maven badge (#132)

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/atnos-org/eff.png?branch=master)](https://travis-ci.org/atnos-org/eff)
 [![Join the chat at https://gitter.im/atnos-org/eff](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/atnos-org/eff?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Maven Central](https://img.shields.io/maven-central/v/org.atnos/eff_2.12.svg)](https://maven-badges.herokuapp.com/maven-central/org.atnos/eff_2.12)
 
 Extensible effects are an alternative to monad transformers for computing with effects in a functional way.
 This library is based on the "free-er" monad and extensible effects described in
@@ -22,7 +23,8 @@ You can also check out [this presentation](http://bit.ly/eff_flatmap_2016) at fl
 
 You add `eff` as an sbt dependency:
 ```scala
-libraryDependencies += "org.atnos" %% "eff" % "4.5.0"
+// check maven badge above for latest version
+libraryDependencies += "org.atnos" %% "eff" % "5.1.0"
 
 // to write types like Reader[String, ?]
 addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.4")