README: explain how to add user methods to PAGE API
> OCR-D framework - file format APIs and schemas
See https://github.com/OCR-D/core
+
+## Adding user methods to the generated PAGE API
+
+Let's say you want to add a method `get_FirstTextRegion` on the pc:Page element:
+
+1. Create a file `ocrd_models/ocrd_page_user_methods/get_FirstTextRegion.py`
+
+```python
+def getFirstTextRegion(self)
+ return self.get_TextRegion[0]
+```
+
+2. Edit `ocrd_models/ocrd_page_user_methods.py` and append to the `METHOD_SPECS` list:
+
+```python
+METHOD_SPECS = (
+ # ...
+ _add_method(r'^PageType$', 'get_TextRegion')
+ # ...
+)
+```
+
+3. Regenerate the PAGE API:
+
+```sh
+make generate-page
+```
