Add documentation on addSimpleGraticule

[dswdejonge]

I missed documentation on the function addSimpleGraticule() in the documentation (the online 'tutorial' for using leaflet in R). I added a small section on this function in the file morefeatures.rmd and added a note about this change in the NEWS file.

I ran devtools::check (no warnings, errors, messages) but did not update documentation or the html file that can be knitted from the rmd file. This is my first ever pull request, and did not feel confident to commit the changes made to the package by my personal machine by the devtools::check command.

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