elaboarate on Google Cloud Platform MQTT sample code

[netskink]

What does this implement/fix? Explain your changes.

This is a fix for this issue
#118

Does this close any currently open issues?

Yes. It closes the above issue. The existing documentation needs elaboration.

Any relevant logs, error output, etc?

None

Any other comments?

This shows that the genkey.sh script creates a key for the device and not the topic. It also shows screenshots of the google cloud platform setup pages and gives detail on how the topic name 'events' seems to be required. Without this name, only pushes work and the device configuration pull in the final part of the code fails.

Where has this been tested?

• lopy4 and exapansion board 3.1
• (sysname='LoPy4', nodename='LoPy4', release='1.20.2.rc6', version='v1.20.1.r2-114-ge264b4c24-dirty on 2020-04-21', machine='LoPy4 with ESP32', lorawan='1.0.2', sigfox='1.0.1')

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}