Update Ffmpeg README with supported NDK versions disclaimer.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190751660

extensions/ffmpeg/README.md

@@ -29,7 +29,8 @@ EXOPLAYER_ROOT="$(pwd)"
 FFMPEG_EXT_PATH="${EXOPLAYER_ROOT}/extensions/ffmpeg/src/main"
 ```
 
-* Download the [Android NDK][] and set its location in an environment variable:
+* Download the [Android NDK][] and set its location in an environment variable.
+  Only versions up to NDK 15c are supported currently.
 
 ```
 NDK_PATH="<path to Android NDK>"