add new postgres resource example

postgresql/postgres.tf

@@ -0,0 +1,19 @@
+terraform {
+  required_providers {
+    gridscale = {
+      source  = "gridscale/gridscale"
+      version = "~> 1.9.0"
+    }
+  }
+}
+
+resource "gridscale_postgresql" "terra-postgres-test" {
+  name = "test"
+  # The PostgreSQL release of this instance. For convenience, please use gscloud (https://github.com/gridscale/gscloud) to get the list of available postgreSQL service releases.
+  release = "13"
+  # Performance class of postgreSQL service. Available performance classes at the time of writing: `standard`, `high`, `insane`, `ultra`.
+  performance_class = "standard"
+  # Maximum CPU core count. The PostgreSQL instance's CPU core count will be autoscaled based on the workload. 
+  # The number of cores stays between 1 and `max_core_count`.
+  max_core_count = 20
+}