Added comment

Author: antoninkriz

src/main/java/org/schabi/newpipe/extractor/stream/SubtitlesFormat.java

@@ -6,6 +6,7 @@ public enum SubtitlesFormat {
     // YouTube subtitles formats
     // TRANSCRIPT(3) is default YT format based on TTML,
     // but unlike VTT or TTML, it is NOT W3 standard
+    // TRANSCRIPT subtitles are NOT supported by ExoPlayer, only VTT and TTML
     VTT (0x0, "vtt"),
     TTML (0x1, "ttml"),
     TRANSCRIPT1 (0x2, "srv1"),