provider/openstack: Notes for using OpenStack with Rackspace (#6205)

website/source/docs/providers/openstack/index.html.markdown

@@ -66,6 +66,48 @@ The following arguments are supported:
     service catalog. It can be set using the OS_ENDPOINT_TYPE environment
     variable. If not set, public endpoints is used.
 
+## Rackspace Compatibility
+
+Using this OpenStack provider with Rackspace is not supported and not
+guaranteed to work; however, users have reported success with the
+following notes in mind:
+
+* Interacting with instances has been seen to work. Interacting with
+all other resources is either untested or known to not work.
+
+* Use your _password_ instead of your Rackspace API KEY.
+
+* Explicitly define the public and private networks in your
+instances as shown below:
+
+```
+resource "openstack_compute_instance_v2" "my_instance" {
+  name = "my_instance"
+  region = "DFW"
+  image_id = "fabe045f-43f8-4991-9e6c-5cabd617538c"
+  flavor_id = "general1-4"
+  key_pair = "provisioning_key"
+
+  network {
+    uuid = "00000000-0000-0000-0000-000000000000"
+    name = "public"
+  }
+
+  network {
+    uuid = "11111111-1111-1111-1111-111111111111"
+    name = "private"
+  }
+}
+```
+
+If you try using this provider with Rackspace and run into bugs, you
+are welcomed to open a bug report / issue on Github, but please keep
+in mind that this is unsupported and the reported bug may not be
+able to be fixed.
+
+If you have successfully used this provider with Rackspace and can
+add any additional comments, please let us know.
+
 ## Testing and Development
 
 In order to run the Acceptance Tests for development, the following environment