Update NEWS.md

add news items for #409, #415, and #459

Author: jgabry

NEWS.md

@@ -1,12 +1,19 @@
 # Items for next release
 
+* New vignette on AB testing. (#409)
+
+* `stan_jm()` gains an offset term for the longitudinal submodel. (#415, @pamelanluna)
+
 * Fix bug where `loo()` with `k_threshold` argument specified would error if the model formula was a string instead of a formula object. (#454)
 
 * Fix bug where `loo()` with `k_threshold` argument specified would error for
 models fit with `stan_polr()`. (#450)
 
 * Fix bug where `stan_aov()` would use the wrong `singular.ok` logic. (#448)
 
+* Fix bug where contrasts info was dropped when subsetting the model matrix in
+`stan_glm()`. (#459)
+
 
 # rstanarm 2.21.1