README: bound the icon exception to its criterion

The previous wording named freesailarr as a documented exception but closed on
"the field is omitted, lint still exits 0", which reads as a general licence to
drop the icon. The rule is now stated with its criterion and its reach: the
exception exists because the chart has no single upstream whose logo the icon
could be, it is the only chart in the repo without one, and a single-upstream
chart missing an icon is a missing icon rather than an exception.

README.md

@@ -247,7 +247,7 @@ Reference implementation: `template-chart/values.schema.json` and `template-char
 
 ### 13. Everything else
 
-- `icon:` is mandatory in every `Chart.yaml`. **Documented exception — freesailarr**: it bundles nine independently versioned projects, so every other chart's rule (the icon is its one upstream's logo) has no answer here, and picking one of the nine would imply a "main app" the stack does not have. The field is omitted; `helm lint --strict` reports `icon is recommended` as INFO and still exits 0.
+- `icon:` is mandatory in every `Chart.yaml`. **Documented exception — freesailarr, the only chart without one**: the rule everywhere else is "the icon is this chart's one upstream's logo", and a chart bundling nine independently versioned projects has no such upstream — picking one of the nine would imply a "main app" the stack deliberately does not have. That is the whole criterion, and it applies to no other chart in the repo: a single-upstream chart without an icon is a missing icon, not an exception. The omission costs an INFO line in `helm lint --strict` (`icon is recommended`), which still exits 0.
 - `helm lint --strict` must pass without findings.
 - `NOTES.txt` for every chart is planned (#43); seven charts ship one so far (`apache-tika`, `cyberchef`, `freesailarr`, `gotenberg`, `homeassistant`, `patchmon`, `rss-bridge`).
 - CI: every installable chart has fixtures under `ci/<chart>/` (`test-values.yaml`, optionally `fixtures.yaml`/`settings.env`) or a justified entry in `ci/excluded-charts.txt` — the install-test fails if both are missing.