Update README with new dependency versioning approach (#578)

As discussed in https://gitea.com/gitea/helm-chart/issues/572.

Also added a bit more context and updates to the overall "Dependencies" sections.

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/578
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

README.md

@@ -5,6 +5,9 @@
 - [Introduction](#introduction)
 - [Update and versioning policy](#update-and-versioning-policy)
 - [Dependencies](#dependencies)
+  - [HA Dependencies](#ha-dependencies)
+  - [Non-HA Dependencies](#non-ha-dependencies)
+  - [Dependency Versioning](#dependency-versioning)
 - [Installing](#installing)
 - [High Availability](#high-availability)
 - [Configuration](#configuration)
@@ -81,10 +84,38 @@ Yet most often no issues will be encountered and the chart maintainers aim to co
 Forgejo can be run with an external database and cache.
 This chart provides those dependencies, which can be enabled, or disabled via configuration.
 
-Dependencies:
+### HA Dependencies
 
-- PostgreSQL HA ([configuration](#postgresql))
+These dependencies are enabled by default:
-- Redis Cluster ([configuration](#cache))
+
+- PostgreSQL HA ([Bitnami PostgreSQL-HA](https://github.com/bitnami/charts/blob/main/bitnami/postgresql-ha/Chart.yaml))
+- Redis-Cluster ([Bitnami Redis-Cluster](https://github.com/bitnami/charts/blob/main/bitnami/redis-cluster/Chart.yaml))
+
+### Non-HA Dependencies
+
+Alternatively, the following non-HA replacements are available:
+
+- PostgreSQL ([Bitnami PostgreSQL](postgresql](https://github.com/bitnami/charts/blob/main/bitnami/postgresql/Chart.yaml)))
+
+### Dependency Versioning
+
+Updates of sub-charts will be incorporated into the Gitea chart as they are released.
+The reasoning behind this is that new users of the chart will start with the most recent sub-chart dependency versions.
+
+**Note** If you want to stay on an older appVersion of a sub-chart dependency (e.g. PostgreSQL), you need to override the image tag in your `values.yaml` file.
+In fact, we recommend to do so right from the start to be independent of major sub-chart dependency changes as they are released.
+There is no need to update to every new PostgreSQL major version - you can happily skip some and do larger updates when you are ready for them.
+
+We recommend to use a rolling tag like `:<majorVersion>-debian-<debian major version>` to incorporate minor and patch updates for the respective major version as they are released.
+Alternatively you can also use a versioning helper tool like [renovate](https://github.com/renovatebot/renovate).
+
+Please double-check the image repository and available tags in the sub-chart:
+
+- [PostgreSQL-HA](https://hub.docker.com/r/bitnami/postgresql-repmgr/tags)
+- [PostgreSQL](https://hub.docker.com/r/bitnami/postgresql/tags)
+- [Redis Cluster](https://hub.docker.com/r/bitnami/redis-cluster/tags)
+
+and look up the image tag which fits your needs on Dockerhub.
 
 ## Installing