Adding an explicit OpenSource Software Copyright Licence, e.g. Apache License 2

[svanteschubert]

If someone would like to reuse your software or parts of the source code, I learned in the past that developers have to take a look at the license.

I suggest easing reuse as much as possible by using a none viral license.
While the choice of license is for some people the choice of religion, I enjoy copying/pasting sub-functionality from existing libraries, which I am maintaining, therefore I would suggest the "Apache Licence 2" license:
https://choosealicense.com/licenses/apache-2.0/
Apache License 2 (AL2) Contributors provide an express grant of patent rights, which aligns with our expectations written in the README to align with UN/CEFACT policy on Intellectual Property Rights.
AL2 also appears to large businesses as stated in https://opensource.guide/legal/ (you might want to gaze at the Table of Contents).

In my (ODF Toolkit) project (which was once donated to Apache, but went away due to the lack of developers), we have

1. A LICENSE file at the project root directory, https://github.com/tdf/odftoolkit/blob/master/LICENSE
2. A license header at the top of every source file (I really do hope that a root LICENSE file is sufficient, but this was requested 20 years ago)
3. A license section at the end of the README.md of the root project: https://github.com/tdf/odftoolkit/#license
4. In addition, at Apache Foundation we had to list the copyright of all 3rd party software in the NOTICE file at the project root level.

In addition, we are using in the project the Apache Maven rat plugin to identify source code, which does not have a license header (as a kind of regression test) and in the pom.xml there is a list of exceptions for files without a header: https://github.com/tdf/odftoolkit/blob/master/pom.xml#L390

I would already be happy with the root LICENSE file (I also suggest copying a section at the end of the README), the rest become if not legally required (as your lawyer) and quite time-consuming and is to me the last 20% of the 100% :-)

In any case, what every license you may choose, all previous contributors have to agree on it!!

[nissimsan]

@svanteschubert , can you open a PR on this, pls? That way we can have it properly reviewed before merging.