Add scope warning to plugins development doc

docs/plugins/development.md

@@ -12,6 +12,9 @@ Plugins can do a lot, including:
 
 However, keep in mind that each piece of functionality is entirely optional. For example, if your plugin merely adds a piece of middleware or an API endpoint for existing data, there's no need to define any new models.
 
+!!! warning
+    While very powerful, the NetBox plugins API is necessarily limited in its scope. The plugins API is discussed here in its entirety: Any part of the NetBox code base not documented here is _not_ part of the supported plugins API, and should not be employed by a plugin. Internal elements of NetBox are subject to change at any time and without warning. Plugin authors are **strongly** encouraged to develop plugins using only the officially supported components discussed here and those provided by the underlying Django framework so as to avoid breaking changes in future releases. 
+
 ## Initial Setup
 
 ## Plugin Structure