Update README.md (#719)

Link to instructions of how to contribute a data set to Tidy Tuesday under "How to participate".

README.md

@@ -21,6 +21,7 @@ If you are using TidyTuesday to teach data-related skills, [please let us know](
 - [Explore the data](https://r4ds.hadley.nz/), watching out for interesting relationships. We would like to emphasize that you should not draw conclusions about **causation** in the data. There are various moderating variables that affect all data, many of which might not have been captured in these datasets. As such, our suggestion is to use the data provided to practice your data tidying and plotting techniques, and to consider for yourself what nuances might underlie these relationships.
 - Create a visualization, a model, a [shiny app](https://shiny.posit.co/), or some other piece of data-science-related output, using R or another programming language.
 - [Share your output and the code used to generate it](sharing.md) on social media with the #TidyTuesday hashtag.
+- **BONUS**: Contribute a data set for a future Tidy Tuesday. Here's [how](https://github.com/rfordatascience/tidytuesday/blob/master/data/curated/template/instructions.md).
 
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