Define funding in package.json

Author: remcohaszing

package.json

@@ -14,6 +14,7 @@
     "url": "https://github.com/remcohaszing/remark-mdx-frontmatter/issues"
   },
   "homepage": "https://github.com/remcohaszing/remark-mdx-frontmatter#readme",
+  "funding": "https://github.com/sponsors/remcohaszing",
   "keywords": [
     "frontmatter",
     "markdown",