Add a release header (#961)

This got dropped in the move to cli_pkg.

Closes #944

tool/grind.dart

@@ -34,6 +34,17 @@ void main(List<String> args) {
   pkg.npmReadme = _readAndResolveMarkdown("package/README.npm.md");
   pkg.standaloneName = "dart-sass";
 
+  pkg.githubReleaseNotes =
+      "To install Sass ${pkg.version}, download one of the packages below and "
+      "[add it to your PATH][], or see [the Sass website][] for full "
+      "installation instructions.\n"
+      "\n"
+      "[add it to your PATH]: https://katiek2.github.io/path-doc/\n"
+      "[the Sass website]: https://sass-lang.com/install\n"
+      "\n"
+      "# Changes\n"
+      "\n";
+
   pkg.addAllTasks();
   grind(args);
 }