Add documentation about java package scanning

[terriko]

Mostly for @anthonyharrison

Currently, the documentation says it can't find anything that doesn't have a checker if you're using the binary scanner. We should probably amend that to mention the .jar file metadata reader, and make sure we describe the support for .jar files in the manual,

[anthonyharrison]

@terriko Agree. Will create a new section called Language Parsers to describe Python, Java and Javascript (see #1548) and also to provide a hook for the GSOC project on more language scanners (#1526)