The :build alias resolves Jackson Core/Databind transitively from
tools.build's Maven resolver at 2.12.4. Cheshire 6.x — pulled in by
martian when codox-md requires midas.client during doc generation —
calls JsonFactoryBuilder.streamReadConstraints which only exists in
Jackson 2.15+. codox-md adds the project's deps via DynamicClassLoader,
which is parent-first, so the older Jackson on the build JVM's
classloader wins and cheshire fails to initialise. The 0.4.0 and 0.5.0
JARs were silently published without docs/midas.client.md as a result.

Pin jackson-core / jackson-databind 2.18.3 in the :build alias so
cheshire's required API is on the parent classloader.

Also adds CONTRIBUTING.md (adapted from clj-gridx) and a Contributing
section in README pointing to it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>