LICENSES: Add the CC-BY-4.0 license

Add the full text of the CC-BY-4.0 license to the kernel tree as well as
the required tags for reference and tooling.

The license text was copied directly from the following url, but for
clarification a 'Creative Commons' was added before 'Attribution 4.0
International' in the first line:
https://creativecommons.org/licenses/by/4.0/legalcode.txt

CC-BY-4.0 is GPLv2 compatible, but when for example used for the
kernel's documentation it can easily happen that sphinx during
processing combines it with text or code from files using a more
restrictive license[1]. This bears pitfalls, hence point that risk out
and suggest to only use this license in combination with the GPLv2.

[1] https://lkml.kernel.org/r/[email protected]

Signed-off-by: Thorsten Leemhuis <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
CC: Thomas Gleixner <[email protected]>
CC: Greg Kroah-Hartman <[email protected]>
CC: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/7115b6c20ae3e6db0370fe4002dd586011205e1c.1607063223.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <[email protected]>