Update README to use gradle annotation processor instead of android-apt

The readme states to use ``apply plugin: 'com.neenbedankt.android-apt'``

"The Android plugin annotation processor works with both javac and jack, unlike android-apt which only works with javac. For more info you can refer to the docs (scroll down a bit on that page)."
see http://www.littlerobots.nl/blog/Whats-next-for-android-apt/

https://bitbucket.org/hvisser/android-apt/wiki/Migration

Open the README and locate the setup instructions containing `com.neenbedankt.android-apt`. Read the linked migration guidance for the intended Gradle annotation-processor wording, then update the README so it no longer instructs users to apply android-apt.

Written by the indexing model from the issue text.