Add KDC for subdomain (sub.test.gokrb5)

[joanlopez]

It adds a new KDC for subdomain (sub.test.gokrb5), with established cross-realm trust with the main realm (test.gokrb5), so it helps reproducing an scenario as the one described in jcmturner/gokrb5#130, as follows:

   test.gokrb5 <--- trust ---> resdom.gokrb5
         /                          \
sub.test.gokrb5           HTTP/host@resdom.gokrb5
       /                             
 testuser1@sub.test.gokrb5    

It also contains a few minor fixes on README.md files.

PS: Note I also added a few comments inline.

[joanlopez]

Closing in favor of #2