gip: expand the README to inlcude explanations

Signed-off-by: Nagy Károly Gábriel <k@jpi.io>

README.md

@@ -1,3 +1,8 @@
 # gip
 
 Find your public IP
+gip will print your public IPv4 followed by a tab and your public IPv6.
+If either is missing they will be replaced by "". Please note that due 
+to their way usage the IPv4 will most probably be the public IP of your
+gateway while the IPv6 will be the public IP of the computer gip is run 
+on.