docs: improve Helm README (existing postgresql Secret) (#3991)

Small doc improvements:
- Add the 2 missing parameters postgresql.existingSecret and postgresql.existingSecretKey in the list.
- Update the "Using an external PostgreSQL server" part by specifying how to use an existing postgresql Secret

***NO_CI***

dev/helm/README.md

@@ -107,6 +107,8 @@ The following table lists the configurable parameters of the Wiki.js chart and t
 | `postgresql.postgresqlUser` | Postgres username | `postgres` |
 | `postgresql.postgresqlHost` | External postgres host | `nil` |
 | `postgresql.postgresqlPassword` | External postgres password | `nil` |
+| `postgresql.existingSecret` | Provide an existing `Secret` for postgres | `nil` |
+| `postgresql.existingSecretKey` | The postgres password key in the existing `Secret` | `postgresql-password` |
 | `postgresql.postgresqlPort` | External postgres port | `5432` |
 | `postgresql.ssl` | Enable external postgres SSL connection | `false` |
 | `postgresql.ca` | Certificate of Authority path for postgres | `nil` |
@@ -137,11 +139,11 @@ By default, PostgreSQL is installed as part of the chart.
 
 ### Using an external PostgreSQL server
 
-To use an external PostgreSQL server, set `postgresql.enabled` to `false` and then set `postgresql.postgresqlHost` and `postgresql.postgresqlPassword`. The other options (`postgresql.postgresqlDatabase`, `postgresql.postgresqlUser` and `postgresql.postgresqlPort`) may also want changing from their default values.
+To use an external PostgreSQL server, set `postgresql.enabled` to `false` and then set `postgresql.postgresqlHost` and `postgresql.postgresqlPassword`. To use an existing `Secret`, set `postgresql.existingSecret`. The other options (`postgresql.postgresqlDatabase`, `postgresql.postgresqlUser`, `postgresql.postgresqlPort` and `postgresql.existingSecretKey`) may also want changing from their default values.
 
 To use an SSL connection you can set `postgresql.ssl` to `true` and if needed the path to a Certificate of Authority can be set using `postgresql.ca` to `/path/to/ca`. Default `postgresql.ssl` value is `false`.
 
-You also need to add the follow Helm template to your deployment:
+If `postgresql.existingSecret` is not specified, you also need to add the following Helm template to your deployment in order to create the postgresql `Secret`:
 
 ```yaml
 kind: Secret