README: add information about which iptables modules are needed.

As requested by a user.

README.md

@@ -30,6 +30,17 @@ common case:
     TCP-over-TCP, which has terrible performance (see below).
     
 
+Prerequisites
+-------------
+
+ - sudo, su, or logged in as root on your client machine
+   (the server doesn't need admin access).
+   
+ - Linux+iptables on your client machine, including at
+   least the iptables DNAT, REDIRECT, and ttl modules. 
+   (This is available by default on most Linux distributions.)
+
+
 This is how you use it:
 -----------------------