Update multiple proguard files import

[marsicdev]

Updates documentation how multiple proguard files can be included in a cleaner way and configured for all build types and flavors.

[enginebai]

When using this, an error occurred when gradle sync. (the same error #138)

[marsicdev]

@enginebai this is known bug for latest android gradle plugin v2.2.2 because of some changes in the api. I updated pull request. You can use the following syntax to fix this:

proguardFiles fileTree(include: ['*.pro'], dir: 'proguard').asList().toArray()

Make sure that your proguard folder is in you application module root directory next to src folder and .gradle file