Update README about both OCR engines in tesseract 4

README.md

@@ -12,6 +12,12 @@
 ## About
 
 This package contains an **OCR engine** - `libtesseract` and a **command line program** - `tesseract`.
+Tesseract 4 adds a new neural net (LSTM) based OCR engine which is focused
+on line recognition, but also still supports the legacy Tesseract OCR engine of 
+Tesseract 3 which works by recognizing character patterns. Compatibility with 
+Tesseract 3 is enabled by using the Legacy OCR Engine mode (--oem 0). 
+It also needs traineddata files which support the legacy engine, for example 
+those from the tessdata repository.
 
 The lead developer is Ray Smith. The maintainer is Zdenko Podobny.
 For a list of contributors see [AUTHORS](https://github.com/tesseract-ocr/tesseract/blob/master/AUTHORS)