gitdm: Add entry for example.com

We already have one instance of it being used in our git history,
and more are probably bound to show up eventually.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/gitdm/groups/unaffiliated

+# This domain will show up because of a mistake, and for that reason we
+# can't really pin it to a specific company or community, so here it is :)
+
+example.com
+
 # These are all domains you can get a personal email address from, so it's
 # fair to assume people using such addresses are contributing in their spare
 # time rather than on behalf of their respective employers.