The nfs.service file below advertises the path to some share. The string:

/path/to/some/share

has to be replaced with the path to the shared folder over NFS.

nfs.service

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name replace-wildcards="yes">%h</name>
  <service>
    <type>_nfs._tcp</type>
    <port>2049</port>
    <txt-record>path=/path/to/some/share</txt-record>
  </service>
</service-group>