automates application of licence header to source files with license-maven-plugin

[abelsromero]

Thank you for opening a pull request and contributing to asciidoctor-maven-plugin!

What kind of change does this PR introduce? (check at least one)

• Bugfix
• [] Feature
• Documentation
• Refactor
• Build improvement
• Other (please describe)

What is the goal of this pull request?
Add & automate copyright to all source files.
Original intention changed as per discusion below, now:

• Add copyright notice to readme.
• Remove unnecessary licence headers. All those that comply with license set at project level.

Are there any alternative ways to implement this?
First, there's al anternative plugin (https://www.mojohaus.org/license-maven-plugin) but I found it seems less mantained.
Secondly, the year could be automated using <maven.build.timestamp.format>yyyy</maven.build.timestamp.format>, but I am afraid of possible side effects during release.
No

Are there any implications of this pull request? Anything a user must know?
No

Is it related to an existing issue?

• Yes
• No

Finally, please add a corresponding entry to CHANGELOG.adoc

[coveralls]

Coverage remained the same at 82.702% when pulling 24b50f2 on abelsromero:feature/automate-application-of-licence-header-to-source-files into 5a7430d on asciidoctor:master.

[abelsromero]

@mojavelinux I used the header of the gradle-plugin as reference but based on other recent experiences, I wonder if the copyright should be the Asciidoctor project instead of current the original author or authors.

[abelsromero]

After reviewing the main repos that include copyright notice (asciidoctor, asciidoctor.js, asciidoctor-intellij-plugin, gradle-plugin, asciidoctor-browser-extension and asciidoctor-reveal.js), I will apply the formula:

Copyright © 2013-YYYY the individual contributors and the Asciidoctor Project.

Honestly, if I have to put specific names I don't know where to begin with, so many great ppl.

[mojavelinux]

Technically, you don't need license headers on every source file. You only need it at the root of the project. That applies to all files in the repository. The only reason you need it per file is if that file carries a different license or needs to be reattributed.

Generally the pattern I have used is that the original creator of the project and the current maintainer (if different) get mentioned in the attribution and then we say "individual contributors" which is a reference to the commits in the history.