docs: deprecate verifyClient

[adrianhopebailie]

I submitted #1612 to update verifyClient because I was unaware of #377 (comment)

This discussion has been going on since 2014 so I figured it would be useful to update some of the examples and docs to save someone else from making the mistake I did 😄

This change:

• Updates the README with a client auth example
• Updates the existing express-sessions example to stop using verifyClient
• Put notes in the docs to discourage the use of verifyClient

[lpinca]

Thank you for this, rubber stamp LGTM. There are a few nits I would like to address. I will specify them later.

[lpinca]

Thank you.