Update README and main.py to clarify optional page range arguments

main.py

@@ -15,8 +15,8 @@ def build_parser() -> argparse.ArgumentParser:
         description="Recognize text from selected PDF pages and save to a text file.",
     )
     parser.add_argument("--input", required=True, type=Path, help="Path to source PDF file.")
-    parser.add_argument("--start", required=True, type=int, help="Start page (1-based, inclusive).")
-    parser.add_argument("--end", required=True, type=int, help="End page (1-based, inclusive).")
+    parser.add_argument("--start", type=int, help="Start page (1-based, inclusive). Default: 1.")
+    parser.add_argument("--end", type=int, help="End page (1-based, inclusive). Default: last page.")
     parser.add_argument("--output", required=True, type=Path, help="Path to output TXT file.")
     parser.add_argument("--lang", default="rus+eng", help="Tesseract language(s), example: rus+eng.")
     parser.add_argument("--dpi", default=300, type=int, help="Render DPI before OCR. Default: 300.")
@@ -37,6 +37,17 @@ def validate_args(args: argparse.Namespace) -> int:
     if input_path.suffix.lower() != ".pdf":
         raise ValueError(f"Input file must have .pdf extension: {input_path}")
 
+    total_pages = get_total_pages(input_path)
+
+    if args.start is None and args.end is None:
+        args.start = 1
+        args.end = total_pages
+    else:
+        if args.start is None:
+            args.start = 1
+        if args.end is None:
+            args.end = total_pages
+
     if args.start < 1:
         raise ValueError("--start must be >= 1.")
 
@@ -50,7 +61,6 @@ def validate_args(args: argparse.Namespace) -> int:
     if args.rotate not in allowed_rotations:
         raise ValueError("--rotate must be one of: 0, 90, 180, 270, -90, -180, -270.")
 
-    total_pages = get_total_pages(input_path)
     if args.end > total_pages:
         raise ValueError(
             f"--end ({args.end}) is out of range. Document has {total_pages} pages.",