Improve documentation

These models don't work with old versions of Tesseract.

Signed-off-by: Stefan Weil <[email protected]>

Author: stweil

README.md

@@ -2,6 +2,9 @@
 
 This repository contains the best trained models for the
 [Tesseract Open Source OCR Engine](https://github.com/tesseract-ocr/tesseract).
+
+These models only work with the LSTM OCR engine of Tesseract 4.
+
 See the [Tesseract wiki](https://github.com/tesseract-ocr/tesseract/wiki/Data-Files)
 for additional information.