Add META.json file for PGXN distribution.

This file will allow you to distribute pgbson on [PGXN](http://pgxn.org/). See the [howto](http://manager.pgxn.org/howto) for details on distributing to PGXN. Once you have created an account and it has been approved, you just have to zip up your repo:

    git archive --format zip --prefix=pgbson-0.5.0 --output pgbson-0.5.0.zip master

And then upload `pgbson-0.5.0.zip` to [PGXN Manager](http://manager.pgxn.org/).

Looking forward to seeing it there!