added Meterpreter-via-SSH payload (#27)

payloads/library/remote-access/Meterpreter-via-SSH/README.md

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+# Meterpreter-via-SSH
+
+## Overview
+This payload starts Packet Squirrel in NAT mode and awaits for user input. When the button is pressed, the payload connects to a remote SSH server and creates a local port tunnel. It then launches a meterpreter shell over said tunnel.
+
+The intent is to get a meterpreter shell on a target network in a way that hides meterpreter network traffic behind legitimate SSH activity.
+
+## Operational Design Considerations
+* Payload remains silent on the network until user presses the button.
+* Payload stops the SSH connection if meterpreter shell fails.
+* Payload always keeps only 1 copy of SSH+meterpreter processes running (even if the button is pressed many times).
+
+## Getting Started
+Copy the payload to Packet Squirrel into desired switch folder, then edit the script to configure your server options:
+* SSH_USER - username on remote SSH server
+* SSH_HOST - ip/domain of remote SSH server
+
+In case you choose to change the default meterpreter port, don't forget to change it on the metasploit side as well.
+* MSF_PORT - port of meterpreter listener
+
+### Generate SSH Key on Squirrel
+You will likely have to generate an ssh key-pair (use default location and empty password) on your Packet Squirrel:
+```
+root@squirrel:~# ssh-keygen
+```
+### Allow Squirrel on SSH Server
+Then you will need to copy the contents of /root/.ssh/id_rsa.pub from Packet Squirrel to the SSH server authorized file:
+```
+user@server:~# mkdir ~/.ssh
+user@server:~# echo 'paste id_rsa.pub contents inside this quote' > ~/.ssh/authorized_keys
+```
+### Run Metasploit with Resource 
+```
+msf@server:~# msfconsole -r server.rc
+```
+
+## LED Definitions
+1. Configure NETMODE 
+* Solid Magenta
+2. Connect to SSH Server
+* SUCCESS - Blink Amber 5 Times
+* FAIL - Blink Red 2 Times
+3. Launch meterpreter
+* SUCCESS - Blink Cyan 1 Time
+* FAIL - Blink Red 1 Time
+
+## Hardening Recommendations
+1. Use an account with limited privileges for SSH access on the server.
+2. Use a dedicated account for Packet Squirrel device (audit usage with SSH access logs).
+3. Disable PasswordAuthentication in sshd_config on the server.