Add instructions for RStudio

How-to-enable-stylistic-sets.md

@@ -43,6 +43,16 @@ To avoid possible bugs, you must uninstall Fira Code 1.x before installing Fira
 }
 ```
 
+# RStudio
+
+Create or edit an [`rstheme` file](https://rstudio.github.io/rstudio-extensions/rstudio-theme-creation.html#testing-changes-to-a-theme) and add:
+
+```css
+.ace_editor {
+  font-feature-settings: "ss01", "ss02", "ss03", "ss04", "ss05", "ss06", "zero", "onum";
+}
+```
+
 # Some Apple apps
 
 ![](https://user-images.githubusercontent.com/285292/64178997-0f98b780-ce6b-11e9-8091-34eab83ca288.png)