Add lecture on Swarm Secrets via CLI

[BretFisher]

From Joao in Discord:

Hi Bret, if you see this I just wanted to ask, since you mentioned you are updating some videos now, if you could update also the video 20 "Using Secrets in Swarm Services" on "Section 4: Swarm Basic Features and How to Use Them In Your Workflow".

In the Q&A there are a couple of suggestions on how to avoid entering secrets that can be revisited in the bash history (starting the command with a whitespace), as well how to avoid trailing newline characters while using echo (using printf instead).

• [ ] Add a written lecture on passwords-in-shell workarounds to Docker Mastery
• [ ] Add a written lecture on passwords-in-shell workarounds to Swarm Mastery