Expose ExcelWriter as part of the Generated API

[WillAyd]

It has a docstring already (though it could use some examples) and would cover usage questions like the below:

https://stackoverflow.com/questions/46173710/where-can-i-find-the-excelwriter-documentation

[gorbondiga]

Hey WillAyd, I am trying to star contributing to pandas, I think this ticket could be a good start point for me, could you assign it to me?

[WillAyd]

We don't use the assignment feature in GitHub but feel free to work on it

[gfyoung]

@WillAyd : I use it from time to time as a way to get pinged should PR's or issues fall to the way side (@jreback at times will do a cleaning of stale issues / PR's). But in this case, @gorbondiga , no need to assign it to you. It's clear from the conversation history that you wanted to work on this first 🙂

[illegalnumbers]

Is this still needing work?

[gfyoung]

@illegalnumbers : Indeed, it does! Go for it, if you want.

[illegalnumbers]

I'll see if I can find some time in the next couple days!