docs: Add note about using previous: replaced and temporary service failures

[richm]

Add a note to the README about the use of previous: replaced and that it can cause temporary service outages to the node being managed. https://github.com/linux-system-roles/firewall/issues/138

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3abd32d) 53.39% compared to head (f86e074) 53.39%.

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