Added 2022 roadmap and migrated previous roadmap from kubeflow/common

[terrytangyuan]

Previous roadmap for 2020 and 2021: https://github.com/kubeflow/common/blob/master/ROADMAP.md

The roadmap for 2022 is still in the early draft. We can continue discussing them going forward.

/cc @kubeflow/wg-training-leads

[google-oss-prow]

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[coveralls]

Pull Request Test Coverage Report for Build 1545564253

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 27.524%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	75.22%

Totals
Change from base Build 1529522486:	0.1%
Covered Lines:	1682
Relevant Lines:	6111

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💛 - Coveralls

[johnugeorge]

/lgtm