chore: update README.md internally

PiperOrigin-RevId: 853872742

Author: aabmass

README.md

@@ -1,5 +1,14 @@
 # Google Cloud Python profiling agent
 
+<!-- copybara:strip_begin(b/450518759) -->
+IMPORTANT: The Cloud Profiler Python agent is deprecated in google3 and no
+longer builds with Python 3.13+ (see b/329431499 for initial deprecation
+discussion and b/450518759 for Python 3.13+ build issues). It is no longer used
+in google3 and TAP for it is disabled. The code remains in google3 as it is the
+source of truth for development before syncing to GitHub; Kokoro tests still
+run as presubmits for GitHub.
+<!-- copybara:strip_end -->
+
 Python profiling agent for
 [Google Cloud Profiler](https://cloud.google.com/profiler/).