add documentation

Author: hrvolapeter

website/docs/index.html.markdown

@@ -133,3 +133,4 @@ The following arguments are supported:
     * `command` - (Required) Command to execute.
     * `args` - (Optional) List of arguments to pass when executing the plugin.
     * `env` - (Optional) Map of environment variables to set when executing the plugin.
+* `proxy_url` - (Optional) URL of the proxy to use for calls.