docs: add Alternatives section referencing bpmn-to-code testing

[emaarco]

What

Adds a compact Alternatives section to the README, cross-referencing Miragon/bpmn-to-code and its process testing.

Why

Clarifies where bpmnlint fits versus where another tool is the better choice:

• bpmnlint — lints one model at a time; the right fit for anything stylistic or about the correctness of a single diagram (naming, layout, structural soundness, engine deployability).
• bpmn-to-code / process testing — engine- and automation-specific questions that span a running process (e.g. does a call activity pass every variable its child process needs), and keeping such tests alongside the model in the same build (e.g. a Gradle task).

The single-model claim was verified against bpmnlint's Linter.lint(moddleRoot, config), which operates on a single parsed model root with no cross-model view.