Merge pull request #870 from hashicorp/f-exec-driver-error-doc-update

Add a specific entry for the common error you may see

website/source/docs/drivers/exec.html.md

@@ -52,6 +52,10 @@ You must specify a `command` to be executed. Optionally you can specify an
 `artifact_source` to be downloaded as well. Any `command` is assumed to be present on the 
 running client, or a downloaded artifact.
 
+If you are receiving the error `* Constraint "missing drivers" filtered <> nodes`
+and using the exec driver, check to ensure that you are running Nomad as root. This
+also applies for running Nomad in -dev mode.
+
 ## Examples
 
 To run a binary present on the Node: