added detailed found useful during RB5 summit

--------------

Whatever the build tool is, binary JVM artifacts are generally published in artifact repositories

that use the Maven2 Repository Format (using groupId/artifactId/version coordinates)

like [Maven Central](https://search.maven.org/) or [Google's Android Repository](https://dl.google.com/dl/android/maven2/index.html).

Being able to rebuild artifacts published in these repositories will require some information: where to get the sources from,

build-tool=<mvn|sbt|...>

build.setup=<optional url of documentation explaining specific additional setup when necessary: will be enhanced in a future buildinfo format version>

# effective build environment information

java.version=<full Java version>

# effective recorded build environment information

java.version=<full Java version taken from "java.version" system property>

java.vendor=<full Java version taken from "java.vendor" system property>

os.name=<Operating system name>

source.used=<artifact|url|scm, depending on which has been used for the build>

sbt.version=1.2.3

scala.version=2.12.6

# and Maven could add data on rebuild instructions and effective environment:

# and Maven could add data on rebuild instructions and effective recorded environment:

mvn.rebuild-args=-Dmaven.test.skip package

mvn.build-root=<groupId>:<artifactId>:<version>

mvn.version=3.5.4

mvn.version=Apache Maven 3.6.3 (cecedd343002696d0abb50b32b541b8a6ba2883f)

# A buildinfo file can contain checksums for multiple output files, for

# example for the main jar and the accompanying pom.xml (when generated):

outputs.0.filename=<file name>

outputs.0.filename=<file name in the repository, ${artifactId}-${version}[-${classifier}].${extension}>

outputs.0.length=<file size>

outputs.0.checksums.sha512=<sha512>

outputs.1.filename=<file name>

outputs.0.checksums.sha512=<sha512 lowercase>

outputs.1.filename=<file name in the repository>

outputs.1.length=<file size>

outputs.1.checksums.sha512=<sha512>

outputs.1.checksums.sha512=<sha512 lowercase>

...

```

Notice that `${artifactId}-${version}-sources.jar` files published in Maven repositories are not buildable sources, but [sources for IDEs](https://central.sonatype.org/pages/requirements.html#supply-javadoc-and-sources).

Source tarballs, intended for building, are not always published in repositories but only sometimes, with 2 classical naming conventions:

- `${artifactId}-${version}-source-release.zip` (see [artifacts in Central providing such source tarballs](https://search.maven.org/search?q=l:source-release))

- `${artifactId}-${version}-src.zip` (see [artifacts in Central providing such source tarballs](https://search.maven.org/search?q=l:src))