Update README.md

FOLLOWING YOUR INSTRUCTIONS PRODUCES THE FOLLOWING ERROR
iMac:gitprojects martinschiller$ git clone git@github.com:MartinRSchiller/BWT.git
fatal: destination path 'BWT' already exists and is not an empty directory.
iMac:gitprojects martinschiller$ git clone git@github.com:MartinRSchiller/BWT.git
Cloning into 'BWT'...
The authenticity of host 'github.com (204.232.175.90)' can't be established.
RSA key fingerprint is 16:27:ac:a5:76:28:2d:36:63:1b:56:4d:eb:df:a6:48.
Are you sure you want to continue connecting (yes/no)? y
Please type 'yes' or 'no': yes
Warning: Permanently added 'github.com,204.232.175.90' (RSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.