Add a NEWS.md entry.

NEWS.md

@@ -9,6 +9,7 @@ If you read this from a place other than <https://mc-stan.org/projpred/news/inde
 * If an L1 search selects an interaction term before all involved lower-order interaction terms (including main-effect terms) have been selected, the predictor ranking is now automatically modified so that the lower-order interaction terms come before this interaction term. A corresponding warning is thrown, which may be deactivated by setting the global option `projpred.warn_L1_interactions` to `FALSE`. Previously, beginning with version 2.5.0, only a warning was thrown and this only if an L1 search selected an interaction term before all involved *main-effect* terms had been selected. (GitHub: #420)
 * Added a progress bar for `project()` (when using the built-in divergence minimizers). For this, `project()` has gained a new argument `verbose` which can also be controlled via the global option `projpred.verbose_project`. By default, the new progress bar is activated. (GitHub: #421)
 * Added a new argument `parallel` to `cv_varsel()`. With `parallel = TRUE`, costly parts of **projpred**'s cross-validation (CV) can be run in parallel. See the documentation of that new argument (and section "Note" of `cv_varsel()`'s documentation) for details. (GitHub: #422)
+* Throw a warning for issue #323 (for multilevel Gaussian models, the projection onto the full model can be instable). (GitHub: #426)
 
 ## Bug fixes