Add META.json for PGXN release.

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=meta-0.1.0 --output meta-0.1.0.zip master

And then upload `meta-0.1.0.zip` to [PGXN Manager](http://manager.pgxn.org/).

You will need to edit the maintainers and license. I also suggest adding a permanent file to point to (rather than the current use of `meta--0.1.sql`).

Looking forward to seeing it there!