Support the homebrew install and update way on Mac OSX

[gadmyth]

If don't use docker, and want integrate the pgmq into an existing postgresql db for my project only have one postgresql db resource.

Because the docker create a new and empty postgresql db which only support the mq function, my project don't want to create another postgresql db.

So how to install the pgmq by homebrew on Mac OSX or by apt-get on ubuntu, or by dnf/yum on fedora ?

So temporarily, I just create the 'pgmq' schema, and execute the src/sql_src.sql script manually, and upgrade later manually too.

[ChuckHend]

Hello @gadmyth . The contributing guide has instructions for installing into an existing Postgres instance which involves installing for source and should work on most operating systems. We recently started releasing deb packages as well, so you could give those a try if you are on linux.

When #230 is complete you'll also be able to install a lot easier if you're on Mac.