Add deprecation message for OD API.

PiperOrigin-RevId: 486695661

Author: Vighnesh Birodkar

research/object_detection/README.md

@@ -3,15 +3,27 @@
 [![TensorFlow 1.15](https://img.shields.io/badge/TensorFlow-1.15-FF6F00?logo=tensorflow)](https://github.com/tensorflow/tensorflow/releases/tag/v1.15.0)
 [![Python 3.6](https://img.shields.io/badge/Python-3.6-3776AB)](https://www.python.org/downloads/release/python-360/)
 
+## Deprecation
+
+*Note to our users*: the Tensorflow Object Detection API is no longer being
+maintained to be compatible with new versions of external dependencies
+(from pip, apt-get etc.). Any changes that follow are meant for internal
+maintenance. We may use the OD API to release projects in the future,
+in which case we will provide full install instructions or Docker images.
+We encourage users seeking an actively maintained detection / segmentation
+codebase to consider [TF-Vision](https://github.com/tensorflow/models/tree/master/official/vision)
+or [scenic](https://github.com/google-research/scenic). We have preserved
+the original install instructions below in case anyone wants to try out old
+models or scripts.
+
 Creating accurate machine learning models capable of localizing and identifying
 multiple objects in a single image remains a core challenge in computer vision.
 The TensorFlow Object Detection API is an open source framework built on top of
 TensorFlow that makes it easy to construct, train and deploy object detection
 models. At Google we’ve certainly found this codebase to be useful for our
 computer vision needs, and we hope that you will as well. <p align="center">
 <img src="g3doc/img/kites_detections_output.jpg" width=676 height=450> </p>
-Contributions to the codebase are welcome and we would love to hear back from
-you if you find this API useful. Finally if you use the TensorFlow Object
+If you use the TensorFlow Object
 Detection API for a research publication, please consider citing:
 
 ```