Update README.txt re: APEZProvider
Update README.txt re: APEZProvider

After struggling with null input streams from an APEZProvider implementation (see https://stackoverflow.com/questions/53997645/solved-why-does-my-content-uri-return-a-null-inputstream-even-though-i-c), I thought it might be helpful to provide further guidance.

While the official docs caution against compressed mp3s and playback media, it could have been clearer that deflated PNGs could run into trouble when trying to grab 'em via "content://" URIs.
googlearchive/play-apk-expansion