Update README.md

Emphasize the plugins.sbt file for snapshot repository, as many people have put it to build.sbt wondering why it doesn't work.
Add 1.3.0 notes
Set version for 1.3.0 release
Use stable version of android-plugin fork dependency
Set next development version
Merge branch 'sbt-0.12' of https://github.com/mpeltonen/sbt-idea into sbt-0.12