add docs about udp tunnels

tunnels.cfg.md

@@ -85,6 +85,37 @@ Optional parameters:
 
 * webircpassword -- password to send with WEBIRC command
 
+UDP Tunnels
+-----------
+
+There are 2 types of UDP tunnels: `udpclient` and `udpserver`
+
+
+`udpclient` forwards 1 local udp endpoint to 1 remote i2p destination
+
+
+    [openvpn-client-simple]
+    type = udpclient
+    destination = something.b32.i2p
+    port = 1194
+
+
+* destination -- the i2p destination of a udpserver tunnel, required parameter
+* address -- ip address to bind local udp endpoint to, defaults to `127.0.0.1`
+* port -- port to bind local udp endpoint to, required parameter
+
+`udpserver` forwards traffic from N i2p destinations to 1 local udp endpoint
+
+    [openvpn-simple-server]
+    type = udpserver
+    keys = openvpn.dat
+    port = 1194
+
+* address -- ip address to use for local udp endpoints, defaults to `127.0.0.1`
+* host -- ip address to forward traffic to, defaults to `127.0.0.1`
+* port -- udp port to forward traffic on, required parameter
+
+
 I2CP parameters
 ---------------