add content readme

cfg/sei_vui/multiplane_image_info/MPIISEI_content_readme.txt

+The video content included is for the purpose of testing the functionality of software to support the multiplane image information SEI message in VTM 
+The video content was derived from MPEG MIV test material CBAbasketball (frame 200-229 of view 07): https://content.mpeg.expert/data/MPEG-I/Part12-ImmersiveVideo/test_material/m58500%20CBAbasketball/
+
+The MPI raw data package can be downloaded from: 
+https://dolby.box.com/s/vp4kuocskiuj5no4ym16g6czbrrhl1cm
+
+The package contains:
+1) MPI texture YUV (896x512, P420, 10bit, 16 layers * 30 frames = 480 YUV frames): cba_tex_896x512_10b_420_std.yuv
+2) MPI opacity YUV (896x512, P400 single channel, 10bit, 16 layers * 30 frames = 480 YUV frames): cba_tra_896x512_10b_400_std.yuv
+3) MPI layer depth array: cba_depth_info.txt
+4) One example cfg file to enable the MPIISEI with VTM encoding: multiplane_image_info_SEI_cba_interleave_example.cfg
+   MPI preprocess need to be done to generate the packed video (follow the MPIISEI_readme.txt) before encoding. This example shows 4x4 layer arrangement and temporal interleaved packing for texture/opacity.