add argument requirements in code example

README.md

@@ -15,8 +15,8 @@ module "pubsub" {
     {
       name                 = "push"   // required
       ack_deadline_seconds = 20 // optonal
-      push_endpoint        = "https://example.com"
-      x-goog-version       = "v1beta1"
+      push_endpoint        = "https://example.com" // required
+      x-goog-version       = "v1beta1" // optional
     }
   ]
   pull_subscriptions = [
@@ -155,4 +155,4 @@ are as follows:
 - Golang - gofmt. gofmt comes with the standard golang installation. golang
 -s a compiled language so there is no standard linter.
 - Terraform - terraform has a built-in linter in the `terraform validate` command.
-- Dockerfiles - hadolint. Can be found in homebrew
+- Dockerfiles - hadolint. Can be found in homebrew