Update readme

README.md

@@ -1,6 +1,8 @@
 # N2N
 
-N2n is a light VPN software which make it easy to create virtual networks bypassing intermediate firewalls. In order to start using N2N, two elements are required:
+N2n is a light VPN software which make it easy to create virtual networks bypassing intermediate firewalls.
+
+In order to start using N2N, two elements are required:
 
 - A *supernode*: it allows edge nodes to announce and discover other nodes. It must have a port publicly accessible on internet.
 
@@ -8,6 +10,8 @@ N2n is a light VPN software which make it easy to create virtual networks bypass
 
 A virtual network shared between multiple edge nodes in n2n is called a *community*. A single supernode can relay multiple communities and a single PC can be part of multiple communities at the same time. An encryption key can be used by the edge nodes to encrypt the packets within their community.
 
+N2n tries to enstablish a direct P2P connection between the edge nodes when possible. When this is not possible (usually due to special NAT devices), the supernode is also used to relay the packets.
+
 Quick Setup
 -----------
 
@@ -118,6 +122,12 @@ Security considerations
 
 n2n edge nodes use twofish encryption by default for compatibility reasons with existing versions.
 
+**IMPORTANT** Encryption is only applied to the packet payload. Some metadata like the virtual MAC address
+of the edge nodes, their IP address and the community are sent in cleartext.
+
+When encryption is enabled, the supernode will not be able to decrypt the traffic exchanged between
+two edge nodes, but it will now that edge A is talking with edge B.
+
 Recently AES encryption support has been implemented, which increases both security and performance,
 so it is recommended to enable it on all the edge nodes by specifying the `-A` option.