update documentation

Author: Pearl1594

website/docs/r/nic.html.markdown

@@ -22,6 +22,17 @@ resource "cloudstack_nic" "test" {
 }
 ```
 
+With MAC address:
+
+```hcl
+resource "cloudstack_nic" "test" {
+  network_id         = "6eb22f91-7454-4107-89f4-36afcdf33021"
+  ip_address         = "192.168.1.1"
+  mac_address        = "02:1a:4b:3c:5d:6e"
+  virtual_machine_id = "f8141e2f-4e7e-4c63-9362-986c908b7ea7"
+}
+```
+
 ## Argument Reference
 
 The following arguments are supported:
@@ -32,6 +43,10 @@ The following arguments are supported:
 * `ip_address` - (Optional) The IP address to assign to the NIC. Changing this
     forces a new resource to be created.
 
+* `mac_address` - (Optional) The MAC address to assign to the NIC. If not specified,
+    a MAC address will be automatically generated. Changing this forces a new resource
+    to be created.
+
 * `virtual_machine_id` - (Required) The ID of the virtual machine to which to
     attach the NIC. Changing this forces a new resource to be created.
 
@@ -41,3 +56,4 @@ The following attributes are exported:
 
 * `id` - The ID of the NIC.
 * `ip_address` - The assigned IP address.
+* `mac_address` - The assigned MAC address.