fix(README): Update instructions to only use kustomize

[Tomcli]

Which issue is resolved by this Pull Request:
Resolves #

Description of your changes:
Updating install instructions to use kubectl -k. kustomize is now a de facto part of kubectl and it make sense to let users install the manifests from it's true source rather than the release yaml we created. That way when master branch is cleaned up, users can still get the source file from each release tag.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/assign @yhwang

[yhwang]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Tomcli, yhwang

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[yhwang]

this change reminds me that we don't need to run those GH actions if the content is doc/readme change only.