add note about handling vpc not found errors.

website/source/intro/getting-started/build-image.html.md

@@ -163,9 +163,11 @@ try to launch the one in the example output above, you will get an error. If you
 want to try to launch your AMI, get the ID from the Packer output.
 
 -> **Note:** If you see a `VPCResourceNotSpecified` error, Packer might not be
-able to 
-might not have a VPC set up. You can either try changing the `instance_type` to
-`m3.medium`, 
+able to determine the default VPC, which the `t2` instance types require. This
+can happen if you created you AWS account before `2013-12-04`.  You can either
+change the `instance_type` to `m3.medium`, or specify a VPC. Please see
+http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/default-vpc.html for more
+information.
 
 ## Managing the Image