Fixed #457 - Inconsistent documentation of minimum required Maven version

[JOpsDev]

• 3.8.1 as been agreed upon to be the minimum for now
• gh-pages need to refreshed from adoc
• added link to contribution guide as well

This checklist will help us to incorporate your contribution quickly and easily:

• Make sure you have read the contributing guide.
• Make sure there is a issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a issue. Your pull request should address just this issue, without
  pulling in other changes.
• Name your branch which contains the pull request accordingly with the issue.
  Best is to follow: issue-Number where Number has be replaced with the issue
  number of the appropriate issue you are offering a pull request for.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like Fixed #Nr - Fixed IT execution.,
  where you replace Nr with the appropriate issue number. It is required
  to use the issue title in the commit message title and in the first line of the
  commit message. They should very similar like the following: Fixed #64 - Automatic site publishing
  You can of course take a look into the existing commit history.
• Make an appropriate entry into the release notes. Follow the examples
  of the existing release notes.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the integration tests successfully (mvn -Prun-its clean verify).

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask via a comment on the appropriate isseu.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• In any other case, please file an Apache Individual Contributor License Agreement.

[khmarbaise]

solved with 470bbb0
Thank you for your contribution.