Manim current status and contribution guidelines

Hello everyone, I will explain the current status of manim in this issue.

Now there are _three_ main manim versions, and their differences:
1. **The `master` branch of [3b1b/manim](https://github.com/3b1b/manim)**: Rendering on GPU using OpenGL and moderngl. Support interaction and have higher efficiency.
2. **[ManimCommunity/manim](https://github.com/ManimCommunity/manim)**: (@ManimCommunity is being improved and will be released on pypi) Using multiple backend rendering. There is better documentation and a more open contribution community.
3. **The [`cairo-backend`](https://github.com/3b1b/manim/tree/cairo-backend) branch of [3b1b/manim](https://github.com/3b1b/manim)**: Rendering on the CPU using cairo. Relatively stable, no longer be developed.

If you want to contribute to manim:
- **The master branch of [3b1b/manim](https://github.com/3b1b/manim)**: See https://3b1b.github.io/manim/development/contributing.html
- **[ManimCommunity/manim](https://github.com/ManimCommunity/manim)** accepts a wider range of new features and pull requests.
- **[manim-kindergarten/manim_sandbox](https://github.com/manim-kindergarten/manim_sandbox)** has a lot of new common classes in the `utils/` folder, and accepts pull requests.

Lastly, thank you very much for using manim and willing to contribute. Enjoy yourself ! 😄