Mention that src/ext should not be edited lightly.

Closes #32661

src/ext/README

@@ -1,3 +1,10 @@
+This directory holds code that was written elsewhere, and is not
+reliably packaged as a library where we want to build, so we ship
+it along with Tor.
+
+In general, you should not edit this code: we are not the maintainers.
+Instead, you should submit patches upstream.
+
 
 OpenBSD_malloc_Linux.c:
 

src/mainpage.md

@@ -84,7 +84,8 @@ The codebase is divided into a few top-level subdirectories, each of
 which contains several sub-modules.
 
    - `ext` -- Code maintained elsewhere that we include in the Tor
-     source distribution.
+     source distribution.  You should not edit this code if you can
+     avoid it: we try to keep it identical to the upstream versions.
 
    - \refdir{lib} -- Lower-level utility code, not necessarily
      tor-specific.