Add note about version numbers

Give a dynamic indication of version numbers, since "0.13" doesn't have an SBT 1.0 equivalent and it took me 30 minutes to figure out it wasn't the repo that was bad, there literally was no artifact there.

Author: wsargent

README.md

@@ -7,9 +7,13 @@ This sbt plugin adds Java agents to projects in a general way. It can enable age
 Add the plugin to your `project/plugins.sbt`:
 
 ```scala
-addSbtPlugin("com.lightbend.sbt" % "sbt-javaagent" % "0.1.3")
+addSbtPlugin("com.lightbend.sbt" % "sbt-javaagent" % versionNumber)
 ```
 
+The version number is determined by the latest package, and can be deduced from the [version.sbt](https://github.com/sbt/sbt-javaagent/blob/master/version.sbt) file, i.e. if the version.sbt is "0.15-SNAPSHOT" then the latest released version is probably "0.14".  You can see the full version packages in the bintray repository:
+
+https://dl.bintray.com/sbt/sbt-plugin-releases/com.lightbend.sbt/sbt-javaagent/scala_2.12/sbt_1.0/
+
 ## Java agent
 
 To add a Java agent to an [sbt-native-packager] distribution, enable the `JavaAgent` plugin on a project that also has `JavaAppPackaging` enabled, and then add the agent dependency using the `javaAgents` setting. For example: