DOC: give read_clipboard a signature

[chris-b1]

Wouldn't necessarily need to add everything that read_table accepts, but at minimum adding a couple common ones like delim_whitespace and sep would make it nicer for argument completion in interactive use.

https://github.com/pydata/pandas/blob/master/pandas/io/clipboard.py#L6

[sinhrks]

Actually it is:) PR is appreciated.