clarify /etc/hosts caveat, needed for local integration tests

the container uses HOSTNAME to generate the cert, and
it really didn't like "localhost" as hostname.

As a workaround, I had to add an arbitrary hostname. There may be other
alternatives to get the host to be known, but the most obvious
is modifying /etc/hosts

Author: vroldanbet

README.rdoc

@@ -59,6 +59,9 @@ Simply run:
   script/ldap-docker
   INTEGRATION=openldap rake test
 
+CAVEAT: you need to add the following line to /etc/hosts
+127.0.0.1 ldap.example.org
+
 == Release
 
 This section is for gem maintainers to cut a new version of the gem.