Add Upstream Release Notes section to the `README.md.gotmpl` so that renovates can link to it consistently

Follow-up work to big-bang/pipeline-templates/renovate-runner#30 (closed) here as part of the "automate renovate tickets" epic. I've been naively patching in an ### Upstream Release Notes section to several READMEs but @akesterson has helpfully pointed out that our gluon-based workflow wipes out those manual changes.

TODO

• add an ### Upstream Release Notes section to the readme template that approximates the content and style of the changes I'd been making by hand
• find a way to layer that content into each repo's Chart.yaml or some other source that our chosen jnorwood/helm-docs template renderer can access at doc generation time

References

• Mattermost code review where my mistake was first identified: mattermost!254 (merged)
• renovate-runner#30 tracking the work that led to this
• Automate Renovate Ticket Workflow epic that triggered thsi work