examples/echo: recommend -secio

When not using -secio, this command prints a warning. This confuses users so we
should just tell users to use it in the instructions.

fixes #179

examples/echo/README.md

@@ -20,7 +20,7 @@ From `go-libp2p` base folder:
 ## Usage
 
 ```
-> ./echo -l 10000
+> ./echo -secio -l 10000
 2017/03/15 14:11:32 I am /ip4/127.0.0.1/tcp/10000/ipfs/QmYo41GybvrXk8y8Xnm1P7pfA4YEXCpfnLyzgRPnNbG35e
 2017/03/15 14:11:32 Now run "./echo -l 10001 -d /ip4/127.0.0.1/tcp/10000/ipfs/QmYo41GybvrXk8y8Xnm1P7pfA4YEXCpfnLyzgRPnNbG35e -secio" on a different terminal
 2017/03/15 14:11:32 listening for connections
@@ -31,7 +31,7 @@ The listener libp2p host will print its `Multiaddress`, which indicates how it c
 Now, launch another node that talks to the listener:
 
 ```
-> ./echo -l 10001 -d /ip4/127.0.0.1/tcp/10000/ipfs/QmYo41GybvrXk8y8Xnm1P7pfA4YEXCpfnLyzgRPnNbG35e
+> ./echo -secio -l 10001 -d /ip4/127.0.0.1/tcp/10000/ipfs/QmYo41GybvrXk8y8Xnm1P7pfA4YEXCpfnLyzgRPnNbG35e
 ```
 
 The new node with send the message `"Hello, world!"` to the listener, which will in turn echo it over the stream and close it. The listener logs the message, and the sender logs the response.
@@ -42,7 +42,7 @@ The `makeBasicHost()` function creates a [go-libp2p-basichost](https://godoc.org
 
 In order to create the swarm (and a `basichost`), the example needs:
 
-- An [ipfs-procotol ID](https://godoc.org/github.com/libp2p/go-libp2p-peer#ID) like `QmNtX1cvrm2K6mQmMEaMxAuB4rTexhd87vpYVot4sEZzxc`. The example autogenerates a key pair on every run and uses an ID extracted from the public key (the hash of the public key). When using `-secio`, it uses the key pair to encrypt communications.
+- An [ipfs-procotol ID](https://godoc.org/github.com/libp2p/go-libp2p-peer#ID) like `QmNtX1cvrm2K6mQmMEaMxAuB4rTexhd87vpYVot4sEZzxc`. The example autogenerates a key pair on every run and uses an ID extracted from the public key (the hash of the public key). When using `-secio`, it uses the key pair to encrypt communications (otherwise, it leaves the connections unencrypted).
 - A [Multiaddress](https://godoc.org/github.com/multiformats/go-multiaddr), which indicates how to reach this peer. There can be several of them (using different protocols or locations for example). Example: `/ip4/127.0.0.1/tcp/1234`.
 - A [go-libp2p-peerstore](https://godoc.org/github.com/libp2p/go-libp2p-peerstore), which is used as a address book which matches node IDs to the multiaddresses through which they can be contacted. This peerstore gets autopopulated when manually opening a connection (with [`Connect()`](https://godoc.org/github.com/libp2p/go-libp2p/p2p/host/basic#BasicHost.Connect). Alternatively, we can manually [`AddAddr()`](https://godoc.org/github.com/libp2p/go-libp2p-peerstore#AddrManager.AddAddr) as in the example.