README should include "wolfictl" command to create an advisory

[tstromberg]

I hate having to figure the right incantation each time.

[luhring]

Thanks for the feedback! Also PRs welcome!

I won't argue that there's as much documentation as there should be. At the same time, there's some existing documentation worth checking out:

• Help information from running wolfictl advisory -h
• Help information from running wolfictl create -h
• Running wolfictl advisory create in the context of resolving a security issue: https://github.com/wolfi-dev/os/blob/main/HOW_TO_PATCH_CVES.md