Add DD3D to the list of external projects.
Summary:
Adding a link to DD3 (https://github.com/facebookresearch/detectron2/commit/f0137779ff5180e56d942898a6677fc2aab082ab)D project to the list of external projects in `project/README.md`.
Pull Request resolved:
@@ -44,4 +44,5 @@ External projects in the community that use detectron2:
+ [FsDet](https://github.com/ucbdrive/few-shot-object-detection), Few-Shot Object Detection.
+ [Sparse R-CNN](https://github.com/PeizeSun/SparseR-CNN)
+ [BCNet](https://github.com/lkeab/BCNet), a bilayer decoupling instance segmentation method.
++ [DD3D](https://github.com/TRI-ML/dd3d), A fully convolutional 3D detector.
