ENH: implement fill_value for df.add(other=Series) #13488
- [ ] closes #13488 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

All in all, I'd rather deprecate support for fill_value and ask users to do `fillna` on their own; I don't think the code complication is worth it @jbrockmendel Thank you for reviewing this PR.  I'd be happy to try a PR for the deprecation of support for fill_value.  I'm new to open source (this is my first contribution) so I'd appreciate it if you could let me know whether this deprecation is worth spending my time on.  Otherwise, I'll select another open Pandas issue to work on.   @jonathanrhughes welcome!  I would not advise doing that deprecation right away since it has not been agreed upon.  Your best bet is to look at issues with the "good first issue" tag (skip the first page or two of results, as those have gotten crowded).
 > All in all, I'd rather deprecate support for fill_value and ask users to do `fillna` on their own; I don't think the code complication is worth it

I would agree with this

> @jonathanrhughes welcome! I would not advise doing that deprecation right away since it has not been agreed upon. Your best bet is to look at issues with the "good first issue" tag (skip the first page or two of results, as those have gotten crowded).

And this. Let's see what others think But in any case @jonathanrhughes thanks for pushing this PR. Very impressive for a first time contribution! @WillAyd I appreciate the feedback.  Thank you.   Well its been 10 days and I haven't seen anything in support of this so going to close. But in any case thanks again @jonathanrhughes - the PR is impressive just not something we have an appetite for