Added subsurface-android-downloader README

The info can be found at subsurface-android-downloader directory.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Updated root README file

Added subsurface-android-qt, subsurface-android-jni and
subsurface-android-downloader descriptions

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>