doc: add note on weakness of permission model

Malicious JavaScript code can bypass the permission model. Hence, it
does not fulfill the requirements of a security mechanism against
malicious code.

Author: tniessen

doc/api/permissions.md

@@ -9,6 +9,15 @@ with those resources.
   The resource can be entirely allowed or denied, or actions related to it can
   be controlled. For example, file system reads can be allowed while denying
   writes.
+  This feature does not protect against malicious code. According to the Node.js
+  [Security Policy][], Node.js trusts any code it is asked to run.
+
+The permission model implements a "seat belt" approach, which prevents trusted
+code from unintentionally changing files or using resources that access has
+not explicitly been granted to. It does not provide security guarantees in the
+presence of malicious code. Malicious code can bypass the permission model and
+execute arbitrary code without the restrictions imposed by the permission
+model.
 
 If you find a potential security vulnerability, please refer to our
 [Security Policy][].