Update license headers

[keilw]

To make this a community effort and reflect the true nature of contributors, we should apply a similar pattern as Red Hat, MicroProfile etc.

Unlike the actual JSR where the Spec Lead is Credit Suisse until it may change some day, here the legal entities can simply be @atsticks, @otaviojava, myself or others.

So we should address this in the license headers, too (the READMe file already does)

[atsticks]

Can you provide a header example?

[keilw]

It's here https://github.com/JavaMoney/javamoney-lib/blob/master/src/etc/header.txt
Not sure if we wanted to add others? the only notable contribution here came from @manuela-grindei