Postgres Permissions Article

Hi All,

I wrote a blog post about the Postgres permissions system, and I thought 
I'd share:

http://illuminatedcomputing.com/posts/2017/03/postgres-permissions/

The main point I wanted to convey, which I somehow never grasped 
confidently from reading the docs or other articles, is how permissions 
are purely additive. Also I wanted to gather in one place how to *see 
the current permissions*, which seems to be missing/incomplete in many 
other articles. Anyway, maybe it will be helpful for some of you! Or for 
the true experts, if you see any errors, I'd be happy to know so I can 
correct them.

I also shared a few opinions amidst the facts (like that `USAGE` for 
schemas doesn't add much), so I am very pleased to have those 
challenged. You can consider them my own outstanding questions. I'd be 
especially grateful for any feedback there.

Yours,
Paul