Add Mac Homebrew CLI installation instructions

Installing Postgres took a while for some people in FACN1. There's around an hour dedicated to installation, so shortening the time to a few minutes ideally would be great.

I've created some (copy/paste friendly) bash Mac CLI instructions:
```bash
(
# install postgresql
brew install postgresql

# initialise database
initdb /usr/local/var/postgres

# start the postgres server
brew services start postgresql

# Get user input for database name
echo 'Please input your desired database name, such as mydb'
read PSQL_DB_NAME

# Get logged in username
PSQL_MY_USERNAME=$(whoami)

# create your database where $PGSQL_DB_NAME is your desired database name and create database with your username to allow using just 'psql'
createdb $PSQL_DB_NAME
createdb $PSQL_MY_USERNAME
);
```