[stable/cert-manager] update with expanded docs. Remove creating TPR support.

[munnerz]

This PR:

• updates the cert-manager Helm chart with the latest changes from upstream
  • namely, removing support for creating TPRs on Kubernetes 1.6 - as it's not actually an operating mode configured nor tested in cert-manager
  • Also due to: Add ability to pass Capabilities.APIVersions to template and lint helm#3377 (more info: Generate and publish deployment manifests for cert-manager cert-manager/cert-manager#276)
• expands the README.md and NOTES.txt with additional post-installation information.

Fixes #3513

[unguiculus]

/ok-to-test

[mattfarina]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattfarina, munnerz

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[mattfarina]

@munnerz is there someone else who should be a maintainer along with you? Someone we could put in an OWNERS file you two could review and approve changes for each other?