Add note in documentation about HAS_NDK_V7 option

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

HOWTO

@@ -35,6 +35,7 @@ Basic how to compile VLC on Android platform.
    cd extras/package/android
 - Build the JNI interface for libvlc
    VLC_BUILD_DIR=path_to_vlc_builddir ANDROID_SDK=path_to_android_sdk make
+- Note: Edit jni/Android.mk and locally delete the line containing HAS_NDK_V7 if you are using Android NDK older than v7
 
 - In Eclipse, create a new Android project from existing source by giving the extra/platform/android/vlc-android path.
 - Compile your project using Eclipse (or make will do it with ant if available).