Add text explaining that :cmd exec's a process without a shell

Author: hugoduncan

README.md

@@ -59,10 +59,14 @@ The `clj-ssh.ssh` namespace should be using SSH from functional code.
     (with-connection session
       (let [result (ssh session {:in "echo hello"})]
         (println (result :out)))
-      (let [result (ssh session "/bin/bash" "-c" "ls" "/")]
+      (let [result (ssh session {:cmd "ls"}]
         (println (second result))))))
 ```
 
+The above example shows using `:in` to pass commands to a shell, and using
+`:cmd` to exec a command without a shell. When using `:cmd` you can still pass
+a stream or a string to `:in` to be used as the process' standard input.
+
 ```clj
 (let [agent (ssh-agent {})]
   (let [session (session agent "localhost" {:strict-host-key-checking :no})]