[vte] test: Document the changes made to UTF-8-test.txt

[Christian Persch <chpe src gnome org>]

commit 416c41be417a051b1d20499754aaa0a827e43b6b
Author: Christian Persch <chpe src gnome org>
Date:   Wed Oct 16 12:23:48 2019 +0200

    test: Document the changes made to UTF-8-test.txt

 perf/UTF-8-test.txt | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/perf/UTF-8-test.txt b/perf/UTF-8-test.txt
index db73cb79..47038dcb 100644
--- a/perf/UTF-8-test.txt
+++ b/perf/UTF-8-test.txt
@@ -3,6 +3,12 @@ UTF-8 decoder capability and stress test
 
 Markus Kuhn <http://www.cl.cam.ac.uk/~mgk25/> - 2015-08-28 - CC BY 4.0
 
+[Note: This file has been slightly modified from its upstream original,
+by changing the right margin so that when it is cat(1)ed in a terminal
+emulator whose UTF-8 decoder implements the whatwg Encoding Spec
+[https://encoding.spec.whatwg.org/], the right margin is correctly
+aligned. -- @chpe]
+
 This test file can help you examine, how your UTF-8 decoder handles
 various types of correct, malformed, or otherwise interesting UTF-8
 sequences. This file is not meant to be a conformance test. It does