Update README.md to add reference links

Author: ypresto

README.md

@@ -75,3 +75,9 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 ```
+
+## References for Android Low-Level Media APIs
+
+- http://bigflake.com/mediacodec/
+- https://github.com/google/grafika
+- https://android.googlesource.com/platform/frameworks/av/+/lollipop-release/media/libstagefright