Sample configuration for multiple operating systems.

Here's a configuration that works for linux, windows, and osx. Mabe you want to include it in the readme.

I've seen issue #5 but it won't handle the classifier and the distribution correctly.

```xml

${project.basedir}/src/main/resources/fbs
${project.build.directory}/generated-sources/java
${project.build.directory}/bin/flatc
tar.gz
distribution-linux

platform-windows


windows



${project.build.directory}/bin/flatc.exe
zip
distribution-windows



platform-osx


mac



distribution-osx

org.apache.maven.plugins
maven-jar-plugin



geogig.flatbuffers





org.apache.maven.plugins
maven-dependency-plugin


unpack
generate-sources

unpack




com.github.davidmoten
flatbuffers-compiler
${flatbuffers-compiler.version}
${fbs.executable.package.type}
${fbs.executable.package.distribution}
true
${project.build.directory}








org.codehaus.mojo
exec-maven-plugin



exec

generate-sources



${fbs.executable}
${fbs.sources}

--java
--gen-mutable
-o
${fbs.generated.sources}
Value.fbs
RevisionObject.fbs




org.codehaus.mojo
build-helper-maven-plugin
3.1.0


add-source
generate-sources

add-source



${fbs.generated.sources}







```

Start by reviewing the README and the existing Maven build configuration to determine where a platform-specific sample belongs. Check that the shown profiles select the correct executable, package type, and distribution for Linux, Windows, and macOS; the work is done when the README contains an accurate usable example.

Written by the indexing model from the issue text.