Rewrite README with usage, status and version guidance

[slachiewicz]

Part of codehaus-plexus/.github#58. Same skeleton as the merged pilots — the last of the thirteen.

The old README explained what Modello is quite well, then gave you a command for building its own site. It never showed how to use it.

For a build-time code generator, the plugin configuration is the thing people come for, so that is now in the README:

<plugin>
  <groupId>org.codehaus.modello</groupId>
  <artifactId>modello-maven-plugin</artifactId>
  …
  <goals>
    <goal>java</goal>
    <goal>xpp3-reader</goal>
    <goal>xpp3-writer</goal>
    <goal>xsd</goal>
  </goals>

With the point that each goal is a generator and you pick the ones you need — which is not obvious from a list of ten plugin modules.

Also adds a one-paragraph summary of what can be generated (XML via five APIs, YAML via two, JSON Schema, XSD, xdoc, converters, Velocity templates since 2.1.0), the Java baseline, and the -Prun-its note for contributors.

The MIT licensing note is kept and given its own section — it is the one project here that is not Apache-2.0, and that is worth not burying.

The site-generation command is dropped; publishing is documented once in RELEASING.md.

Ran mvn spotless:apply.