Add note on SSH key

[phamduchongan93]

Inform users about importance of private key on local machine.

1. make sure private key is saved for future need.
2. please generate ssh key with passphrase (for security).

[vmnet8]

Do you mean to emphasis this part in step 2 SSH tutorial ?

"NOTE: With cat ~/.ssh/id_rsa* you can check your SSH key pair. Both parts (private and public) of your recently created SSH key should show up. Please be sure that your (private) key is protected by a passphrase has been created on your computer and not on the Raspberry Pi. "

[phamduchongan93]

@vmnet8 This was intended for @dogi and his associates having a copy of public key that match user's private key. This will act as authentication and authorization for future need when VI works with OLE.