Update release document

[andreyvelich]

I updated the release document.
Eventually, we should automate the release process for Training Operator.

I will create separate issue to track release improvements that we need to follow for Training Operator.

/assign @kubeflow/wg-training-leads

[coveralls]

Pull Request Test Coverage Report for Build 9666312688

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 35.39%

---

Totals
Change from base Build 9543811102:	0.03%
Covered Lines:	4376
Relevant Lines:	12365

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9666410579

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 35.423%

---

Totals
Change from base Build 9543811102:	0.07%
Covered Lines:	4380
Relevant Lines:	12365

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9666507304

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 35.398%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob.go	1	91.06%

Totals
Change from base Build 9543811102:	0.04%
Covered Lines:	4377
Relevant Lines:	12365

---

💛 - Coveralls

[tenzen-y]

Thank you!
/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenzen-y

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [tenzen-y]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment