Add mandatory plugin keyword #3

https://github.com/postcss/postcss/blob/master/docs/guidelines/plugin.md#44-include-postcss-plugin-keyword-in-packagejson

Author: RadValentin

package.json

@@ -20,7 +20,8 @@
   "keywords": [
     "postcss",
     "prefix",
-    "selectors"
+    "selectors",
+    "postcss-plugin"
   ],
   "files": [
     "index.js"