docs(README.md): advise to generate separate client for each user

[jedwards1211]

I made the mistake of giving the same file to multiple users, this seems to be a common mistake (https://serverfault.com/a/461230/360582) at least for people like me who don't RTFM

[JellevanKraaij]

You can just add an option into the server configuration to allow for multiple users on the same file. "duplicate-cn" is it called. I had the same problem before.

[jedwards1211]

Yes I found out about that option, but since it's not default and not necessarily reasonable for all use cases, I think it would be good for the readme to at least guide users away from this pitfall in the default use case