DOC: add hvplot as option to plotting.backend

[raybellwaves]

Location of the documentation

Couple of sections here
https://pandas.pydata.org/pandas-docs/stable/ecosystem.html#visualization
https://dev.pandas.io/docs/development/extending.html#plotting-backends

Documentation problem

Add information that hvplot can be used as a backend
pd.set_option("plotting.backend", "hvplot")

Suggested fix for documentation

Not sure will this could go. Could add a hvplot section to https://github.com/pandas-dev/pandas/blob/master/doc/source/ecosystem.rst#visualization but perhaps want to make an explicit list of all the plotting backend options at https://dev.pandas.io/docs/development/extending.html#plotting-backends

[raybellwaves]

Happy to work on this is if it is recommended where to put it.