Add page for testers.

Testers.md

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+To ensure NewPipe is running on all supported Android versions and different hardware smoothly, we need help with testing new features and changes during development. In particular, we are looking for testers with KitKat devices, Android TVs and manufactures who have their own battery optimization software integrated in the OS.
+
+If you want to help us, please add yourself to the list below. When listed below, we will @mention you when your help is needed. Thank you in advance!
+
+## KitKat
+Old, but gold. We know NewPipe is popular on one of the first widely spread Android OS due to its small APK size and low hardware requirements. We devs don't have KitKat devices and the KitKat AVDs are not that reliable.
+
+- hey! This list is empty. Be the first one on it :P
+
+## Android TV
+We not only need pure Android TVs, but also devices running AOSP on TVs. Simply everything which can be controlled with a remote :)
+
+- If you have a :tv:, your name should be here
+
+## Battery optimized devices
+Devices by Huawei, Samsung (Android P+ and newer), OnePlus and Xiaomi sometimes kill NewPipe when playing in background. We've already created [a support page](https://newpipe.schabi.org/FAQ/tutorials/bugs-huawei-background-player-workaround/) for that and would like to enhance the app to notify users of these devices and provide more info in our FAQ.
+
+- Don't remove NewPipe from your phone, remove this line and add yourself to the list