Add PGXN META.json.

[PGXN](http://pgxn.org/) is an extension distribution network for PostgreSQL. Distributing your extension on PGXN is pretty straight-forward; the only thin it needs is an account and a `META.json` file. Here is the file. You can get the details on creating an account and uploading distributions in the [PGXN HOWTO](http://manager.pgxn.org/howto).