[RFC] Add initial version of the roadmap (#159)

Signed-off-by: terrytangyuan <[email protected]>

Author: terrytangyuan

ROADMAP.md

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+# Roadmap of MPI Operator
+
+This document provides a high-level overview of where MPI Operator will grow in future releases. See discussions in the original RFC [here](https://github.com/kubeflow/mpi-operator/pull/159).
+
+## New Features / Enhancements
+
+* Decouple the tight dependency on Open MPI and support other collective communication frameworks.
+Related issue: [#12](https://github.com/kubeflow/mpi-operator/issues/12).
+* Support new versions of MPI Operator in [kubeflow/manifests](https://github.com/kubeflow/manifests).
+* Redesign different components of MPI Operator to support fault tolerant collective communication frameworks such as [caicloud/ftlib](https://github.com/caicloud/ftlib).
+* Allow more flexible RBAC when `MPIJob`s so existing RBAC resources can be reused. Related issue: [#20](https://github.com/kubeflow/mpi-operator/issues/20).
+* Support installation of MPI Operator via [Helm](https://github.com/helm/helm). Related issue: [#11](https://github.com/kubeflow/mpi-operator/issues/11).
+* Support [Go modules](https://blog.golang.org/migrating-to-go-modules).
+* Consider support launching framework-specific services such as [TensorBoard](https://www.tensorflow.org/tensorboard) and [Horovod Timeline](https://github.com/horovod/horovod#horovod-timeline). Since [tf-operator](https://github.com/kubeflow/tf-operator) already supports TensorBoard, we may want to consider moving this to [kubeflow/common](https://github.com/kubeflow/common) so it can be reused. Related issue: [#138](https://github.com/kubeflow/mpi-operator/issues/138).
+
+## CI/CD
+
+* Automate the process to publish images to Docker Hub whenever there's new release/commit. Related issue: [#93](https://github.com/kubeflow/mpi-operator/issues/93).
+* Ensure new versions of `deploy/mpi-operator.yaml` are always compatible with [kubeflow/manifests](https://github.com/kubeflow/manifests).
+* Add end-to-end tests via Kubeflow's testing infrastructure. Related issue: [#9](https://github.com/kubeflow/mpi-operator/issues/9).
+
+## Bug Fixes
+
+* Better statuses of launcher and worker pods. Related issues: [#90](https://github.com/kubeflow/mpi-operator/issues/90)