Update docs to specify k8s version 1.15 or greater

[ncskier]

Changes

Fixes #517

Triggers now requires kubernetes version 1.15 or higher; this PR updates
the docs to reflect this.

/cc @dibyom
/cc @joe-sonrichard

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• Includes tests (if functionality changed/added)
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Release Notes

Updates the minimum k8s version in the docs to 1.15.

[tekton-robot]

@ncskier: GitHub didn't allow me to request PR reviews from the following users: joe-sonrichard.

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In response to this:

Changes

Fixes #517

Triggers now requires kubernetes version 1.15 or higher; this PR updates
the docs to reflect this.

/cc @dibyom
/cc @joe-sonrichard

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices

See the contribution guide for more details.

Release Notes

Updates the minimum k8s version in the docs to 1.15.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[dibyom]

/approve
/lgtm

[tekton-robot]

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