Added instructions for using --server-user

[cbrachem]

There seems to be some recurring issues with users which either have an RSA SSH key or do not know that they need to use the --server-user parameter when they are using different usernames on the server and the new node they want to add. I have added some info to the readme that will hopefully clear that up a bit.

Signed-off-by: Carsten Brachem [email protected]

Description

Just some additions to the readme regarding the use of the --server-user parameter and adding some common solutions to this issue to the troubleshooting section.

Motivation and Context

• Fixes a part of SSH Error - handshake failed #63 by explaining a CLI parameter and pointing out an underlying issue with RSA keys
• I have raised an issue to propose this change (required) <-- I have not because the issue already existed

How Has This Been Tested?

• Just changes in the readme, tested by reviewing the resulting markup document

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Just documentation

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Thanks for the PR.