Docs: Add warning about prior creation of guard/config.json file

[Diogo-Rego]

What was changed?

• Updated the README.md documentation to include an important warning ([!WARNING]) targeting misp-guard container users.

Motivation

• Prevent Docker or Podman from automatically creating guard/config.json as an empty directory if the file does not exist prior to container startup, which causes container failure and continuous restarts.

How to test / Validate

1. Review the updated section in the README.
2. Run the suggested prior creation commands:

   mkdir -p guard
   touch guard/config.json