Add enable_link_security (#1942)

Author: KDederichs

core/subresources.md

@@ -339,7 +339,7 @@ In order to use Symfony's built-in security system on subresources the security
 
 To restrict the access to a subresource based on the parent object simply use the Symfony expression language as you would do normally, with the exception that the name defined in `toProperty` or `fromProperty` is used to access the object.
 
-Alternatively you can also use the `securityObjectName` to set a custom name
+Alternatively you can also use the `securityObjectName` to set a custom name.
 
 ```php
 <?php 
@@ -357,3 +357,11 @@ class Company {
     // ...
 }
 ```
+
+This is currently an experimental feature disabled by default. To enable it please set `enable_link_security` to true:
+
+```yaml
+# api/config/packages/api_platform.yaml
+api_platform:
+    enable_link_security: true
+```