Suggestions for documentation

[eric-haibin-lin]

It would be great to improve the documentation for first-time users. Current documentation assumes users know horovod APIs, but for users who just get into distributed training they do not necessarily have that prior knowledge/experience.

I suggest BytePS have a section similar to https://github.com/horovod/horovod#usage introducing the high level APIs (DistributedOptimizer) as well as low level APIs (pushpull for a custom tensor).

[bobzhuyb]

We will do a throughout revisit for all the documentation before release 0.2.0. Hopefully 0.2.0 will arrive in Feb.