add deprecation warning when delimiter passed

Author: minggli

pandas/io/parsers.py

@@ -635,6 +635,9 @@ def parser_f(filepath_or_buffer,
         # Alias sep -> delimiter.
         if delimiter is None:
             delimiter = sep
+        else:
+            warnings.warn("delimiter is deprecated, please use sep instead.",
+                          DeprecationWarning, stacklevel=2)
 
         if delim_whitespace and delimiter != default_sep:
             raise ValueError("Specified a delimiter with both sep and"