add keywords

Author: azu

package.json

@@ -36,5 +36,8 @@
     "kuromojin": "^1.3.2",
     "morpheme-match": "^1.0.1",
     "morpheme-match-all": "^1.1.0"
-  }
-}
+  },
+  "keywords": [
+    "textlintrule"
+  ]
+}