Where does plugins instruction go in gradle files?

I'm developing an Android project with Android Studio and gradle. I have seen that from Gradle 2.1+ you can include plugins with the new plugin instruction, like this:

plugins {
  id "me.tatarka.retrolambda" version "3.2.5"
}

Where does this instruction go? In which gradle file do I have to put it, and nested somewhere or just appended at the end?