Add changes file for #13196.

Note that this will likely need to be folded with the changes file for #12751,
as this change is a mere fixup on top of the changes introduced for #12751.

changes/bug13196-systemd-writable-run-directory

@@ -0,0 +1,3 @@
+  o Distribution:
+    - systemd unit file: set up /var/run/tor as writable for the Tor service.
+      Patch by intrigeri; resolves ticket 13196.