Document lack of support for non-UTF8 encodings.

README.md

@@ -183,6 +183,10 @@ Sass to update the reference behavior.
 15. `@extend` doesn't produce an error if it matches but fails to unify. See
     [issue 2250][].
 
+16. Dart Sass currently only supports UTF-8 documents. We'd like to support
+    more, but Dart currently doesn't support them. See [dart-lang/sdk#11744][],
+    for example.
+
 [issue 1599]: https://github.com/sass/sass/issues/1599
 [issue 1126]: https://github.com/sass/sass/issues/1126
 [issue 2120]: https://github.com/sass/sass/issues/2120
@@ -198,5 +202,6 @@ Sass to update the reference behavior.
 [issue 303]: https://github.com/sass/sass/issues/303
 [issue 2247]: https://github.com/sass/sass/issues/2247
 [issue 2250]: https://github.com/sass/sass/issues/2250
+[dart-lang/sdk#11744]: https://github.com/dart-lang/sdk/issues/11744
 
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