added URL for git repository served over git:// protocol

HACKING

@@ -19,6 +19,10 @@ compile the code, you need the following tools:
 
 To check out the master branch, use:
 
+   git clone git://git.strongswan.org/strongswan.git
+
+     or using HTTP
+
    git clone http://git.strongswan.org/strongswan.git
 
 After a successful check out, give the autotools a try: