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/README.md

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+# Test Setup
+
+This directory contains miscellaneous files used to setup a test database.

testsetup/localhost.cnf

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+[ req ]
+default_bits       = 2048
+distinguished_name = dn
+req_extensions     = v3_req
+prompt             = no
+[ dn ]
+commonName         = localhost
+[ v3_req ]
+subjectAltName     = @alt_names
+keyUsage           = digitalSignature
+extendedKeyUsage   = serverAuth
+[alt_names]
+DNS.1              = localhost

testsetup/pg_hba.conf

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+local     all         postgres                          trust
+local     all         all                               trust
+host      all         pgx_md5     127.0.0.1/32          md5
+host      all         pgx_scram   127.0.0.1/32          scram-sha-256
+host      all         pgx_pw      127.0.0.1/32          password
+hostssl   all         pgx_ssl     127.0.0.1/32          scram-sha-256

testsetup/postgresql_setup.sql

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+-- 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';

testsetup/postgresql_ssl.conf

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+ssl = on
+ssl_cert_file = 'server.crt'
+ssl_key_file = 'server.key'
+ssl_ca_file = 'root.crt'