Add release notes item about profiling/debugging features

Co-authored-by: Stephan Hageboeck <stephan.hageboeck@cern.ch>

README/ReleaseNotes/v628/index.md

@@ -73,6 +73,17 @@ $ rootreadspeed --files <local-folder>/File1.root xrootd://<url-folder>/File2.ro
 
 ## Core Libraries
 
+### Support for profiling/debugging interpreted/JITted code
+
+This version of ROOT adds an LLVM JIT event listener to create perf map files
+during runtime. This allows profiling of interpreted/JITted code generated by
+cling. Instead of function addresses, the perf data will contain full function
+names. In addition, stack frame pointers are enabled in JITted code, so full
+stack traces can be generated. Debugging is aided by switching off optimisations
+and adding frame pointers for better stack traces. However, since both have a
+runtime cost, they are disabled by default. Similar to `LD_DEBUG` and `LD_PROFILE`
+for `ld.so`, the environment variables `CLING_DEBUG=1` and/or `CLING_PROFILE=1`
+can be set to enable debugging and/or profiling.
 
 ## I/O Libraries