add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* `geom_ribbon()` now appropriately warns about, and removes, missing values 
+  (@teunbrand, #6243).
 * `guide_*()` can now accept two inside legend theme elements:
   `legend.position.inside` and `legend.justification.inside`, allowing inside
   legends to be placed at different positions. Only inside legends with the same