chore(npm): add "textlintrule" as keywords

textlint/textlint#253

Author: azu

package.json

@@ -28,7 +28,8 @@
     "prepublish": "npm run --if-present build"
   },
   "keywords": [
-    "textlint"
+    "textlint",
+    "textlintrule"
   ],
   "devDependencies": {
     "babel-cli": "^6.10.1",
@@ -43,4 +44,4 @@
   "dependencies": {
     "textlint-rule-helper": "^2.0.0"
   }
-}
+}