Add missing licenses for content material

[rclement]

Some of the event content material are hosted in this repository but do not have explicit licenses.

As suggested by @Lenormju, it would be good to have reusable content for various use-cases (teaching, workshops, etc.). While we have to respect content creators with their own licensing choices (to be defined where they are hosting the material), we could at least agree to apply an open and reusable license for content hosted in this repository.

Here is some documentation about Creative Commons licenses:

• https://creativecommons.org/licenses/
• https://en.wikipedia.org/wiki/Creative_Commons_license#Types_of_licenses
• https://cccc.ncte.org/cccc/committees/ip/2009developments/openeducation

The current idea would be to use CC-BY-SA ("Attribution-ShareAlike") in the same vein as GPL-like copyleft licenses:

• Allow the use in any context (even commercial)
• The original authors must be credited
• Remixed material must be shared under the same terms.