Added some missing helm vars documentation for Agones intall with helm.

[EricFortin]

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

What this PR does / Why we need it:
Documents helm vars used in templates but not listed on the site.

Which issue(s) this PR fixes:

Closes #

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 3f7e3123-216f-4a02-b8be-a8d956fe450b

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

• image: gcr.io/agones-images/agones-controller:1.7.0-fb6e518
• image: gcr.io/agones-images/agones-sdk:1.7.0-fb6e518
• image: gcr.io/agones-images/agones-ping:1.7.0-fb6e518
• Linux C++ SDK (build): agonessdk-1.7.0-fb6e518-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.7.0-fb6e518.zip

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

• https://fb6e518-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1607/head:pr_1607 && git checkout pr_1607
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.7.0-fb6e518

[cyriltovena]

LGTM

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cyriltovena, EricFortin

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