How to add a linked source folder in Android Studio?

in your build.gradle add the following to the end of the android node

android {
    ....
    ....

    sourceSets {
        main.java.srcDirs += 'src/main/<YOUR DIRECTORY>'
    }

}

The right answer is:

android {
    ....
    ....

    sourceSets {
        main.java.srcDirs += 'src/main/<YOUR DIRECTORY>'
    }
}

Furthermore, if your external source directory is not under src/main, you could use a relative path like this:

sourceSets {
    main.java.srcDirs += 'src/main/../../../<YOUR DIRECTORY>'
}

You can add a source folder to the build script and then sync. Look for sourceSets in the documentation here: http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Basic-Project

I haven't found a good way of adding test source folders. I have manually added the source to the .iml file. Of course this means it will go away everytime the build script is synched.