Update HACKING.md add-on instructions.
Update HACKING.md add-on instructions.

Solinea/goldstone-server,slashk/goldstone-server,slashk/goldstone-server,slashk/goldstone-server,Solinea/goldstone-server,Solinea/goldstone-server,slashk/goldstone-server,Solinea/goldstone-server,slashk/goldstone-server,Solinea/goldstone-server