Update README and MQTT security settings to support updated Sling protocol

[RichDom2185]

Related PRs:

• Sling: Add new monitor topic for sending real-time information from devices to clients sling#1
• Frontend: Add real-time dashboard for viewing connected peripherals on the EV3 frontend#2229
• EV3-source (coming soon)

Although the abovementioned PRs are related, this PR is safe to merge independently.

Changes:

• Remove hard breaks in + reformat README (the former was causing rendering issues as they were interpreted as separate lines)
• Improves the developer experience by providing a guide on how to set up a local MQTT server to test remote execution locally
• Update terraform whitelists for MQTT topics to support the updated Sling version (containing a new message topic, monitor)

[coveralls]

Coverage remained the same at 95.997% when pulling 3482eda on RichDom2185:richard/update-readme-and-mqtt-security-settings into b25ebce on source-academy:master.