Add Spark Operator Roadmap

[ChenYi015]

Purpose of this PR

Proposed changes:

• Add ROADMAP.md file as this is needed as CNCF graduation.

Change Category

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that could affect existing functionality)
• Documentation update

Rationale

Checklist

• I have conducted a self-review of my own code.
• I have updated documentation accordingly.
• I have added tests that prove my changes are effective or that my feature works.
• Existing unit tests pass locally with my changes.

Additional Notes

[andreyvelich]

Great work!
/assign @vara-bonthu @nabuskey @jacobsalway @yuchaoran2011 @ImpSy

[andreyvelich]

Thank you @ChenYi015!
/lgtm
/assign @vara-bonthu @nabuskey @jacobsalway

[andreyvelich]

Thank you for this @ChenYi015 🎉
/lgtm
/approve

[google-oss-prow]

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