Tested on FreeBSD 10 and above:
/path/to/somewhere -ro -mapall=nobody -alldirs -network 192.168.1.0/24
nfs_server_enable="YES" nfsv4_server_enable="YES" nfsuserd_enable="YES"
This will share your directory to all clients on that have 192.168.1.X as IP address with read only permissions on your network. Have in mind that all clients don't support v4, OpenELEC for instance only supports up to v3 which you'll need to adjust in your sysctl accordingly.