Add a NEWS.md entry for stan-dev#393.

NEWS.md

@@ -12,6 +12,7 @@ If you read this from a place other than <https://mc-stan.org/projpred/news/inde
 * Reduced dependencies. (GitHub: #388)
 * Argument `digits` of `print.vselsummary()` is not used for an internal `round()` call anymore, but passed to argument `digits` of `print.data.frame()`, meaning that it now determines the minimum number of *significant digits* to be printed. (GitHub: #389)
 * Although bad practice (in general), a reference model lacking an intercept can now be used within **projpred**. However, it will always be projected onto submodels which *include* an intercept. The reason is that even if the true intercept in the reference model is zero, this does not need to hold for the submodels. An informational message mentioning the projection onto intercept-including submodels is thrown when **projpred** encounters a reference model lacking an intercept. (GitHub: #96, #391)
+* In case of non-predictor arguments of `s()` or `t2()`, **projpred** now throws an error. (This had already been documented before, but a suitable error message was missing.) (GitHub: #393, based on #156 and #269)
 * L1 search now throws a warning if an interaction term is selected before all involved main effects have been selected. (GitHub: #395)
 * Due to internal refactoring: