Deprecate fill_value argument for df.add
This came up in the submission for #13488 

This is a documented argument but strangely not universally implemented for all argument types. Both @jbrockmendel and I agreed it is probably better to deprecate and push users towards using something like what @jreback suggested in https://github.com/pandas-dev/pandas/issues/13488#issuecomment-227016181