update CONTRIBUTING.md

CONTRIBUTING.md

@@ -137,3 +137,25 @@ Additional Commands:
   set-rdns                    Change reverse DNS of a Server
   ssh                         Spawn an SSH connection for the server
 ```
+
+### Command validation
+
+Please use the `util.Validate` method to make sure the command is validated against its usage string during runtime.
+This can be done by setting the `Args` field of the `cobra.Command` struct to `util.Validate`:
+
+```go
+cmd := &cobra.Command{
+   Use:   "my-command [options]",
+   Args:  util.Validate,
+   Run: func(cmd *cobra.Command, args []string) {
+      // ...
+   },
+}
+```
+
+If you are using base commands like `base.Cmd` or `base.ListCmd` etc., this is already done for you.
+
+**Note:** Your command usage needs to follow the [docopt](http://docopt.org/) syntax for this to work.
+If your command uses optional positional arguments or other complicated usage that necessitates to disable
+argument count checking, you use `util.ValidateLenient` instead. It will not throw an error if there are
+more arguments than expected.