docs: Update note regarding the jndiReferenceName (#772)

Author: t-ott

README.md

@@ -306,8 +306,7 @@ POSTGRES_PASS=${POSTGRES_PASS}
 POSTGRES_JNDI_NAME=${POSTGRES_JNDI_NAME}
 ```
 
-When defining the parameters for the store in GeoServer you will need to set
-`jndiReferenceName=java:comp/env/jdbc/postgres`
+The `jndiReferenceName` for Postgres (JNDI) stores in GeoServer will be derived from the `POSTGRES_JNDI_NAME` provided. For example, if `POSTGRES_JNDI_NAME=postgres`, set `jndiReferenceName=java:comp/env/jdbc/postgres`.
 
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