docs: add routing-tables to experiment (#898)

Signed-off-by: Mauritz Uphoff <mauritz.uphoff@stackit.cloud>

Author: h3adex

README.md

@@ -183,7 +183,7 @@ To enable experiments set the experiments field in the provider definition:
 ```hcl
 provider "stackit" {
   default_region        = "eu01"
-  experiments           = ["iam"]
+  experiments           = ["iam", "routing-tables"]
 }
 ```
 
@@ -193,6 +193,10 @@ provider "stackit" {
 
 Enables IAM management features in the Terraform provider. The underlying IAM API is expected to undergo a redesign in the future, which leads to it being considered experimental.
 
+#### `routing-tables`
+
+This feature enables experimental routing table capabilities in the Terraform Provider, available only to designated SNAs at this time.
+
 ## Acceptance Tests
 
 Terraform acceptance tests are run using the command `make test-acceptance-tf`. For all services,