Add github topics to repo

I think it would increase the visibility of this repo to add Github topics. I was surprised when I didn't see this repo near the top of https://github.com/topics/image-labeling

Instructions for adding github topics:
https://help.github.com/en/articles/classifying-your-repository-with-topics

example topics from [opencv/cvat](https://github.com/opencv/cvat):

Open the repository's GitHub settings and review the linked topic-classification instructions and the existing topics. Add suitable topics that accurately describe the LabelMe annotation tool and image-labeling focus, then confirm they appear on the repository page and under the relevant GitHub topic search.

索引モデルが issue の本文から書いたものです。