README, scripts: move Build instructions to 'gimp-web-devel'
Depends on: gimp-web-devel!50 (merged)
The other OSes (Linux and Windows) have documentation in the gimp-web-devel site. This is due the fact they are supported by GIMP. macOS is a supported system too but its documentation is "relegated" to a separate place (this repo).
To have better visibility and probably more contributions, would be reasonable to have the build instructions in the gimp-web-devel. This way, macOS is on pair with the other OS in terms of facility for first contribution.