add AGENT.md (#842)

This just hopefully catches a common gotcha we run into in CI in this
repo

### Checklist

- [ ] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

<!--
As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests
are REQUIRED to
provide a "test plan". A test plan is a loose explanation of what you
have done or
implemented to test this, as outlined in our Testing principles and
guidelines:

https://docs.sourcegraph.com/dev/background-information/testing_principles
  Write your test plan here after the "Test plan" header.
-->

Author: DaedalusG

AGENTS.md

@@ -0,0 +1,11 @@
+# Agent Instructions
+
+## After Making Changes
+
+After making changes to any `values.yaml` file, regenerate the helm docs and verify there are no uncommitted changes:
+
+```sh
+./scripts/helm-docs.sh && [[ -z $(git status -s) ]]
+```
+
+If the README was updated, stage and commit the changes alongside your other modifications.