Clarify passwordless SSH-key requirements

[acch]

README currently contains the following:

A pre-requisite for installing IBM Spectrum Scale is that passwordless SSH must be configured among all nodes in the cluster. Passwordless SSH must be configured and checked with FQDN, hostname, and IP of every node to every node.

My understanding is that this is not correct:

• It is required in all cases to authorize the SSH key of the Ansible control node with all managed nodes. This is mandatory.
• Authorizing the SSH keys of all managed nodes among each other can (optionally) be accomplished by running the core/precheck role. Hence, this is optional.

I can change the section and submit a PR... just wanted to open this up for discussion, first, to make sure that I'm not missing something obvious.