The pull request includes instructions on how to configure mqtt server:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/pull/113
Alternatively, there is also a UI pull request that includes mqtt. To do so, download that version of the UI:
https://github.com/OpenSprinkler/OpenSprinkler-App/pull/86
unzip it and double click the ‘index.html’ in ‘www’ folder. It works just like the regular UI, and it shows mqtt server configuration under Options -> Integration.