Document macOS installation requirements

Currently, running tools on Mac OS Catalina fails because we lack the appropriate signatures, sign the wrong way or both. That's tracked under K8S-1395, but for now we need to document the workaround.

Anil Kumar can probably offer details on specific actions required to make it run if you can't just find it on the 'net (it's a common problem).

New Description:

Catalina installation requirements were release-noted here: https://docs.couchbase.com/operator/current/release-notes.html#known-issues-v200

However, the changes made as part of K8S-1395 need to be reviewed for documentation requirements (both on Catalina and Big Sur)