Add simple/concrete example of custom rule package for publishing (fixes #133).

test/rules/npm/sample-rule.js

@@ -0,0 +1,19 @@
+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "sample-rule" ],
+  "description": "Sample rule",
+  "tags": [ "sample" ],
+  "function": function rule(params, onError) {
+    params.tokens.forEach((token) => {
+      if (token.type === "hr") {
+        onError({
+          "lineNumber": token.lineNumber,
+          "detail": "Sample error for hr"
+        });
+      }
+    });
+  }
+};