website: document exec driver

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

@@ -10,5 +10,41 @@ description: |-
 
 Name: `exec`
 
-TODO
+The `exec` driver is used to simply execute a particular command for a task.
+This is the simplest driver and is extremely flexible. In particlar, because
+it can invoke any command, it can be used to call scripts or other wrappers
+which provide higher level features.
+
+## Task Configuration
+
+The `exec` driver supports the following configuration in the job spec:
+
+* `command` - The command to execute. Must be provided.
+
+* `args` - The argument list to the command, space seperated. Optional.
+
+## Client Requirements
+
+The `exec` driver has no special requirements and can run on all
+supported operating systems. The resource isolation primitives vary
+by OS.
+
+## Client Attributes
+
+The `exec` driver will set the following client attributes:
+
+* `driver.exec` - This will always be set to "1", indicating the
+  driver is available.
+
+## Resource Isolation
+
+The resource isolation provided varies by the operating system of
+the client and the configuration.
+
+On Linux, Nomad will attempt to use cgroups, namespaces, and chroot
+to isolate the resources of a process. If the Nomad agent is not
+running as root many of these mechanisms cannot be used.
+
+As a baseline, the task driver will just execute the command
+with no additional resource isolation if none are available.