Expand feature freeze details during RC.

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup
/kind documentation
/kind feature
/kind hotfix

/kind meta

What this PR does / Why we need it:

There have been a few discussions on what PRs get merged during release candidate, which I think is down to the release documentation not being as clear as it could be.

This should make the guidelines for what should go through and what should be held back during release candidate clearer.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, roberthbailey

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• OWNERS [markmandel,roberthbailey]

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[agones-bot]

Build Succeeded 👏

Build Id: f3219162-4d70-4f77-b5c4-0fa6feacde53

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.10.0-df8d44b
• image: gcr.io/agones-images/agones-sdk:1.10.0-df8d44b
• image: gcr.io/agones-images/agones-ping:1.10.0-df8d44b
• Linux C++ SDK (build): agonessdk-1.10.0-df8d44b-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.10.0-df8d44b.zip

A preview of the website (the last 30 builds are retained):

• https://df8d44b-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1847/head:pr_1847 && git checkout pr_1847
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.10.0-df8d44b