text: Title: Will Keras welcome new backend contributions?
  Body: Suppose we could develop a brand-new backend for Keras, such as [Paddle](https://github.com/PaddlePaddle/Paddle). Would Keras welcome our new backend contributions?
  
  In fact, I can understand that adding a new backend will increase the maintenance burden of the keras-team. Therefore, I would like to ask for keras-team's opinion.