Adds an external link for a gene family. Could be useful if gene families are derived from a different
platform, or to link to similar families in other databases.
Example input: four columns are required (tab delimited), the family name, the name in the referenced platform, the name of the platform and the
url.
family01 family01 external_website http://url.com/family01
family02 family02 external_website http://url.com/family02
family03 family03 external_website http://url.com/family03
...