Update link to contributing guide from the membership template.

[roberthbailey]

What type of PR is this?

Uncomment only one /kind <> line, press 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:

Which issue(s) this PR fixes:

Closes #

Special notes for your reviewer:

[agones-bot]

Build Succeeded 👏

Build Id: 6770e95a-7872-4763-ba35-be113dbf94eb

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

• image: gcr.io/agones-images/agones-controller:1.15.0-e849f94
• image: gcr.io/agones-images/agones-ping:1.15.0-e849f94
• Linux C++ SDK (build): agonessdk-1.15.0-e849f94-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.15.0-e849f94.zip

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

• https://e849f94-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2118/head:pr_2118 && git checkout pr_2118
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.15.0-e849f94

[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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