How to enable Multi-Window Mode on Android Marshmallow 6.0

In Android 6.0, there's a hidden feature called multi window. It has a warnings:





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So this is actually for developer. If you want to do, you should back up all data and find the way to back to Stock ROM if any before following these steps

NOTE : 

-Your phone on 6.0 android

-Your phone should be rooted ( no root still