html/charset/testdata: update licensing info in README

Update the licensing info following the instructions at
http://www.w3.org/Consortium/Legal/2008/04-testsuite-copyright.html#howtouse

Fixes golang/go#10398.

Change-Id: Ib37b2a696a5287f41d4e85da4eb7ec1cbb346301
Reviewed-on: https://go-review.googlesource.com/8978
Reviewed-by: Martin Packman <martin.packman@canonical.com>
Reviewed-by: Rob Pike <r@golang.org>

html/charset/testdata/README

@@ -1 +1,9 @@
-These test cases come from http://www.w3.org/International/tests/html5/the-input-byte-stream/results-basics
+These test cases come from
+http://www.w3.org/International/tests/repository/html5/the-input-byte-stream/results-basics
+
+Distributed under both the W3C Test Suite License
+(http://www.w3.org/Consortium/Legal/2008/04-testsuite-license)
+and the W3C 3-clause BSD License
+(http://www.w3.org/Consortium/Legal/2008/03-bsd-license).
+To contribute to a W3C Test Suite, see the policies and contribution
+forms (http://www.w3.org/2004/10/27-testcases).