docs(readme): fetch the OCR models in Get started (refs storytold/artcraft#2104) (#1006)

README.md

@@ -372,6 +372,7 @@ PdfCraft is a clean-room implementation. Its behaviour comes from the ISO 32000
 ```sh
 git clone https://github.com/storytold/pdfcraft
 cd pdfcraft
+cargo xtask models                                # fetch the OCR models Scan & OCR needs (release builds include them)
 cargo run --release -p pdfcraft -- some.pdf     # desktop app
 cargo xtask demo-pdf                              # build the showcase PDF used in these screenshots
 cargo xtask screenshots                           # regenerate every screenshot in this README