I'm glad you've found skrf useful and thanks for being willing to write some documentation (I'd never come across this function myself until you mentioned it). The easiest way to contribute documentation is probably in the form of a jupyter notebook if you are familiar with the format. You can fork the skrf repository using git, write a notebook with your plotting example, and place it in the "scikit-rf\doc\source\examples\plotting" directory. You can then create a pull request on github to add it into the main branch for the next release. It is good practice to clear the output/metadata from the notebook before pushing it to git. You can use nbstripout for this or I think there may be a menu option in the jupyter gui itself.
Hope that helps. If you have any other questions feel free to ask and I'm sure someone will be able to help answer them for you!