chore(npm): add "textlintrule" as keywords

textlint/textlint#253

Author: azu

package.json

@@ -29,7 +29,8 @@
     "prepublish": "npm run --if-present build"
   },
   "keywords": [
-    "textlint"
+    "textlint",
+    "textlintrule"
   ],
   "devDependencies": {
     "babel-cli": "^6.10.1",
@@ -46,4 +47,4 @@
     "morpheme-match": "^1.0.1",
     "textlint-rule-prh": "^3.1.1"
   }
-}
+}