docs: mention temporary workaround for #92

TODO: revert this commit when #92 is fixed.

docs/adding_custom_rest.md

@@ -23,8 +23,15 @@ type Foo struct {
     // Your resource definition here
 }
 
-// Custom REST storage
+// Initialize custom REST storage
 func NewFooREST() rest.Storage {
-    // Your rest.Storage implementation here
+    // Initialize fields of custom REST implementation
 }
-```
+
+// Your rest.Storage implementation below
+// ...
+```
+
+**Warning:** NewFooREST() should not contain any non-trivial logic, besides
+simply initializing the fields of the struct, that represents the custom REST.
+See [this issue](https://github.com/kubernetes-incubator/apiserver-builder/issues/92) for details.