Add roadmap document for xgboost-operator

[terrytangyuan]

Per @abhi-g's comment in kubeflow/internal-acls#234 (comment), let's add a roadmap doc as part of our next planning meeting so others who might be interested can contribute as well.

cc @merlintang @ywskycn @sperlingxx @abhi-g @johnugeorge @richardsliu

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[merlintang]

From my point of view,

1. provide python SDK for distributed dataset loader for easily starting distributed training. now it is not easy for end-users to start the training.
2. support Light GBM or other tree-based approaches for distributed tree training
3. compact with kubeflow common updating.

[merlintang]

For the LGBM support, we leave this a todo work. #56

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