[stable/sentry] Add note about Sentry ownership (#15266)

This PR adds a note to the readme stating that the chart is not maintained or backed by Sentry org itself.

Signed-off-by: Burak Yigit Kaya <byk@sentry.io>

stable/sentry/Chart.yaml

@@ -1,7 +1,7 @@
 apiVersion: v1
 description: Sentry is a cross-platform crash reporting and aggregation platform.
 name: sentry
-version: 1.5.6
+version: 1.5.7
 appVersion: 9.1.1
 keywords:
   - debugging

stable/sentry/README.md

@@ -2,6 +2,8 @@
 
 [Sentry](https://sentry.io/) is a cross-platform crash reporting and aggregation platform.
 
+_This helm chart is **not** official nor maintained by Sentry itself._
+
 ## TL;DR;
 
 ```console