[0x73746F66]

What

Adds a paragraph to the README's development section. It covers just mutation (gremlins v0.6.0, the 80% default threshold, and the ./internal/purl default package), the CI job that runs it ("Mutation testing"), and how to survey another package (just mutation ./internal/versions 0).

Why

#334 added the recipe and the CI job but did not update the README, so contributors had no way to find it.

How verified

• Checked the recipe name, default package, threshold and pinned version against the justfile and the Test Action workflow added in ci: mutation testing with gremlins (just mutation, Test Action job) #334.
• Docs-only change, so no code checks apply. Rebased onto the latest main.

[github-actions]

Vulnetix config plan

Manager gitops:vulnetix/cli, prune off, plan hash a8bd984f1753c68b180298051dd9e2524ab644d2ebe9e4c7c811ff4126753b5f

	Object	Action	Changes
~	QualityGate/default	update (adopt)	severity: null -> "high"

0 to create, 1 to update, 0 to delete, 0 unchanged

[github-actions]

Vulnetix config plan

Manager gitops:vulnetix/cli, prune off, plan hash a8bd984f1753c68b180298051dd9e2524ab644d2ebe9e4c7c811ff4126753b5f

	Object	Action	Changes
~	QualityGate/default	update (adopt)	severity: null -> "high"

0 to create, 1 to update, 0 to delete, 0 unchanged