Improve testing / contributing instructions

* Extract CONTRIBUTING.md
* Add instructions and scripts to setup standalone PostgreSQL server
  that tests the various connection and authentication types.

testsetup/postgresql_setup.sql

@@ -0,0 +1,15 @@
+-- Create extensions and types.
+create extension hstore;
+create domain uint64 as numeric(20,0);
+
+-- Create users for different types of connections and authentication.
+create user pgx_ssl PASSWORD 'secret';
+set password_encryption = md5;
+create user pgx_md5 PASSWORD 'secret';
+set password_encryption = 'scram-sha-256';
+create user pgx_pw PASSWORD 'secret';
+create user pgx_scram PASSWORD 'secret';
+
+-- The tricky test user, below, has to actually exist so that it can be used in a test
+-- of aclitem formatting. It turns out aclitems cannot contain non-existing users/roles.
+create user " tricky, ' } "" \\ test user " superuser password 'secret';