Add back missing libvpx build instructions

PiperOrigin-RevId: 242094027

extensions/vp9/README.md

@@ -34,6 +34,22 @@ VP9_EXT_PATH="${EXOPLAYER_ROOT}/extensions/vp9/src/main"
 NDK_PATH="<path to Android NDK>"
 ```
 
+* Fetch libvpx:
+
+```
+cd "${VP9_EXT_PATH}/jni" && \
+git clone https://chromium.googlesource.com/webm/libvpx libvpx
+```
+
+* Checkout the appropriate branch of libvpx (the scripts and makefiles bundled
+  in this repo are known to work only at specific versions of the library - we
+  will update this periodically as newer versions of libvpx are released):
+
+```
+cd "${VP9_EXT_PATH}/jni/libvpx" && \
+git checkout tags/v1.7.0 -b v1.7.0
+```
+
 * Run a script that generates necessary configuration files for libvpx:
 
 ```
@@ -58,6 +74,10 @@ ${NDK_PATH}/ndk-build APP_ABI=all -j4
   * Android config scripts should be re-generated by running
     `generate_libvpx_android_configs.sh`
   * Clean and re-build the project.
+* If you want to use your own version of libvpx, place it in
+  `${VP9_EXT_PATH}/jni/libvpx`. Please note that
+  `generate_libvpx_android_configs.sh` and the makefiles may need to be modified
+  to work with arbitrary versions of libvpx.
 
 ## Using the extension ##