Pipelines: [Process] Update backend development README

[Bobgy]

We have https://github.com/kubeflow/pipelines/tree/master/backend, but it is highly outdated because we have moved away from bazel.
It'll be great if we can update it to reflect current best practices.

[Bobgy]

Hi @NikeNano, I think you recently developed backend, do you want to contribute to this?

[NikeNano]

Sure would be happy to help out! Will look at it after I sorted out the cron scheduler.

[NikeNano]

/assign