Update readme and add link to deeplab2 (#10233)

research/deeplab/README.md

 # DeepLab: Deep Labelling for Semantic Image Segmentation
 
+**To new and existing DeepLab users**: We have released a unified codebase for
+dense pixel labeling tasks in TensorFlow2 at https://github.com/google-research/deeplab2.
+Please consider switching to the newer codebase for better support. 
+
 DeepLab is a state-of-art deep learning model for semantic image segmentation,
 where the goal is to assign semantic labels (e.g., person, dog, cat and so on)
 to every pixel in the input image. Current implementation includes the following