Add a notice that NDK <= version 15c is required for VP9

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177430827

extensions/vp9/README.md

@@ -28,7 +28,8 @@ EXOPLAYER_ROOT="$(pwd)"
 VP9_EXT_PATH="${EXOPLAYER_ROOT}/extensions/vp9/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 (see [#3520][]).
 
 ```
 NDK_PATH="<path to Android NDK>"
@@ -70,6 +71,7 @@ ${NDK_PATH}/ndk-build APP_ABI=all -j4
 
 [top level README]: https://github.com/google/ExoPlayer/blob/release-v2/README.md
 [Android NDK]: https://developer.android.com/tools/sdk/ndk/index.html
+[#3520]: https://github.com/google/ExoPlayer/issues/3520
 
 ## Notes ##