add citation for my claim about "majority favoring code-unit" view.

active/0000-restrict-xXX-to-ascii.md

@@ -108,10 +108,12 @@ Some reasons not to do this:
 The thesis of this RFC is that the first bullet is a falsehood.
 
 While there is some precedent for the "`\xXX` is code point"
-interpretation in some languages, the majority do seem to favor the
+interpretation in some languages, the [majority] do seem to favor the
 "`\xXX` is code unit" point of view.  The proposal of this RFC is
 side-stepping the distinction by limiting the input range for `\xXX`.
 
+[majority]: https://mail.mozilla.org/pipermail/rust-dev/2012-July/002025.html
+
 The third bullet is a strawman since we have not yet released 1.0, and
 thus everything is up for change.