add voxel r-cnn in readme (#610)

* Update __init__.py

missing kornia. before image_to_tensor

* Update README.md

add Voxel R-CNN to changelog

* Update README.md

Add Voxel R-CNN as the officially released code

README.md

@@ -4,7 +4,7 @@
 
 `OpenPCDet` is a clear, simple, self-contained open source project for LiDAR-based 3D object detection. 
 
-It is also the official code release of [`[PointRCNN]`](https://arxiv.org/abs/1812.04244), [`[Part-A^2 net]`](https://arxiv.org/abs/1907.03670) and [`[PV-RCNN]`](https://arxiv.org/abs/1912.13192). 
+It is also the official code release of [`[PointRCNN]`](https://arxiv.org/abs/1812.04244), [`[Part-A^2 net]`](https://arxiv.org/abs/1907.03670), [`[PV-RCNN]`](https://arxiv.org/abs/1912.13192) and [`[Voxel R-CNN]`](https://arxiv.org/abs/2012.15712). 
 
 
 ## Overview
@@ -18,6 +18,8 @@ It is also the official code release of [`[PointRCNN]`](https://arxiv.org/abs/18
 
 
 ## Changelog
+[2021-06-08] Added support for the voxel-based 3D object detection model [`Voxel R-CNN`](#KITTI-3D-Object-Detection-Baselines)
+
 [2021-05-14] Added support for the monocular 3D object detection model [`CaDDN`](#KITTI-3D-Object-Detection-Baselines)
 
 [2020-11-27] **Bugfixed:** Please re-prepare the validation infos of Waymo dataset (version 1.2) if you would like to