Write libcouchbase cert. auth. example

See recent release notes for 2.8.4 and discuss with Sergey Avseyev as needed to show how to use certificate authentication with libcouchbase.

https://developer.couchbase.com/server/other-products/release-notes-archives/c-sdk

Mention that this as "uncommitted functionality" and subject to potential change. 

This is important to have documented as next CB Server release includes this capability and will need to tested and validated by some end users.