Add PDF save instructions to docs

[skip ci]

Author: int3l

README.rst

@@ -81,6 +81,8 @@ USAGE
 
     # Get a searchable PDF
     pdf = pytesseract.image_to_pdf_or_hocr('test.png', extension='pdf')
+    with open('test.pdf', 'w+b') as f:
+        f.write(pdf) # pdf type is bytes by default
 
     # Get HOCR output
     hocr = pytesseract.image_to_pdf_or_hocr('test.png', extension='hocr')