Add MaskAL to the list of external projects.

[pieterblok]

Hello @ppwwyyxx ,

I've created new active learning software that can significantly reduce the annotation effort for training Mask R-CNN in Detectron2. The code can be found here: https://github.com/pieterblok/maskal

There is also a paper publication on arXiv: https://arxiv.org/abs/2112.06586

Can you please add the github link to the list of external projects in project/README.md file? Like you did 20 days ago with DD3D.

Thanks in advance,
Regards,
Pieter