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

test/rules/npm/package.json

@@ -1,13 +1,13 @@
 {
-  "name": "markdownlint-rules-test",
+  "name": "markdownlint-rule-sample",
   "version": "0.0.1",
-  "description": "Package of markdownlint custom rules used for testing",
-  "main": "rules.js",
+  "description": "Package for markdownlint custom rule sample",
+  "main": "sample-rule.js",
   "author": "David Anson (https://dlaa.me/)",
   "homepage": "https://github.com/DavidAnson/markdownlint",
   "license": "MIT",
   "keywords": [
-    "markdownlint-rules"
+    "markdownlint-rule"
   ],
   "private": true
 }

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"
+        });
+      }
+    });
+  }
+};