Document the environment variables

[EasyXploit]

Actual environment variables:

• NODE_ENV: The environment the bot is running in (development | production)
• LOCALE: The name of the locale to use (see the /locales folder for available locales)
• DISCORD_TOKEN: The token of the bot application provided by Discord
• DB_TARGET: The FQDN or IP address to connect to the MongoDB database on
• DB_PORT: The port to connect to the MongoDB database on
• DB_NAME: The name of the MongoDB database that the bot will use to store data
• DB_AUTH_SOURCE: The name of the MongoDB database to authenticate against
• DB_USERNAME: The username to authenticate with to the MongoDB database
• DB_PASSWORD: The password to authenticate with to the MongoDB database
• SENTRY_DSN: The DSN of the Sentry project
• SENTRY_TRACES_SAMPLE_RATE: The sample rate of traces to send to Sentry (0.0 - 1.0)