docs: more info for README

[StackScribe]

• Note about resources
• Enable KLT
• Move cert-manager install to OF section
• Misc notes

[AloisReitbauer]

We install Keptn with the Helm chart as part of the GitIOps deployment. I think, most of this is not needed anymore

[StackScribe]

We are using the walkthrough files rather than the main README.md file so I am closing this.