Add R notes

Co-authored-by: Bryce Mecum <[email protected]>

_posts/2024-07-16-17.0.0-release.md

@@ -157,6 +157,11 @@ Thanks for your contributions and participation in the project!
 
 ## R notes
 
+* R functions that users write that use functions that Arrow supports in dataset queries now can be used in queries too. Previously, only functions that used arithmetic operators worked. For example, `time_hours <- function(mins) mins / 60` worked, but `time_hours_rounded <- function(mins) round(mins / 60)` did not; now both work. These are automatic translations rather than true user-defined functions (UDFs); for UDFs, see `register_scalar_function()`. [GH-41223](https://github.com/apache/arrow/issues/41223)
+* `summarize()` supports more complex expressions, and correctly handles cases where column names are reused in expressions. [GH-41323](https://github.com/apache/arrow/issues/41323)
+* The `na_matches` argument to the `dplyr::*_join()` functions is now supported. This argument controls whether `NA` values are considered equal when joining. [GH-41223](https://github.com/apache/arrow/issues/41358)
+* R metadata, stored in the Arrow schema to support round-tripping data between R and Arrow/Parquet, is now serialized and deserialized more strictly. This makes it safer to load data from files from unknown sources into R data.frames. [GH-41223](https://github.com/apache/arrow/issues/41969)
+
 For more on what’s in the 17.0.0 R package, see the [R changelog][4].
 
 ## Ruby and C GLib notes