Add changelog

fastlane/metadata/android/en-US/changelogs/900.txt

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+New
+• Subscription groups and sorted feeds
+• Mute button in players
+
+Improved
+• Allow opening music.youtube.com and media.ccc.de links in NewPipe
+• Relocate two settings from Appearance to Content
+• Hide 5, 15, 25 second seek options if inexact seek is enabled
+
+Fixed
+• some WebM videos are not seekable
+• database backup on Android P
+• crash when sharing a downloaded file
+• tons of YouTube extraction issue and more ...