Add a Crash Handling subsection. (#376)

Add some words about the new crash handler, including a promise of a
future blog post on the subject.

Author: al45tair

_posts/2023-08-29-swift-5.9-released.md

@@ -82,6 +82,14 @@ A full list of Swift 5.9's Evolution proposals can be found at the end of this p
 
 <!-- TODO: Add more sections with content? -->
 
+### Crash Handling
+
+On Linux, the Swift runtime will now catch program crashes and Swift runtime errors and display a backtrace on the program's output.  The backtracer is out-of-process and includes support for `async` functions.
+
+This feature is also available on macOS, although there it is disabled by default (it can be enabled by setting `SWIFT_BACKTRACE=enable=yes`) and you need to ensure that your program is signed with the `com.apple.security.get-task-allow` entitlement.
+
+A more detailed blog covering this feature will be published soon.
+
 ### Debugging
 
 In Swift 5.9, we introduced a couple of new features to [LLDB](https://lldb.llvm.org/) and the Swift compiler aimed at making Swift debugging faster and more reliable.