Added info about targetNamespace change in README (#2559)

* Added info about targetNamespace change

* Update README.md

README.md

@@ -143,6 +143,13 @@ For full list of changes, you can [compare tags](https://github.com/OpenMage/mag
 
 [Full list of events](docs/EVENTS.md)
 
+### Changes to SOAP/WSDL
+
+Since `19.4.17`/`20.0.15` we changed the `targetNamespace` of all the WSDL files (used in the API modules), from `Magento` to `OpenMage`.
+If your custom modules extends OpenMage's APIs with a custom WSDL file and there are some hardcoded `targetNamespace="urn:Magento"` string, your APIs may stop working.
+Please replace all occurrences of `targetNamespace="urn:Magento"` with `targetNamespace="urn:OpenMage"` (or alternatively `targetNamespace="urn:{{var wsdl.name}}"`) to avoid any problem.
+To find which files need the modification you can run `grep -rn 'urn:Magento' --include \*.xml` from the root directory of your project.
+
 ## Development Environment with ddev
 
 - Install [ddev](https://ddev.com/get-started/)