Improve Postgresql configuration

[MartinDelille]

As I said on the IRC, I'm always struggling when it comes to configure postgresql. If someone has a good documentation for noobs like me, I'll link it to the home page. I also think the documentation is not very pertinent regarding to my configuration (postgresql install via Homebrew / OSX) and I'll be happy to improve it :-)

[Changaco]

I would like to avoid adding OS-specific instructions to the README. We should modify the first command (sudo -u postgres createuser --superuser $USER) to be more generic (sudo may not be installed, the user may not be called postgres, the $USER variable may not be set). We should also add links to the official postgresql documentation: https://www.postgresql.org/download/ and https://www.postgresql.org/docs/9.4/static/index.html.

Previous discussion: liberapay/salon#7 (comment).

[Changaco]

For comparison, take a look at Gratipay's README.

[MartinDelille]

Thanks I forgot we had this previous discussion! I will try to improve the doc in a more generic way which is totally fine for me!