Add descriptive header license file

[gtkramer]

Add header to license file that contains the following text to make it more clear as to what users are looking at:

# This list is automatically generated.
#
# To read the full license text for these licenses, please visit
# https://spdx.org/licenses. A few license identifiers in this list
# are not declared on the https://spdx.org/licenses website:
# - Copyright
# - Distributable
# - Public-Domain
#
# These are used for projects that have explicitly granted redistribution
# of the project source code, but don't have a typical OSI-approved
# license identifier.

[gtkramer]

@ahkok how does this look? It's slightly different than what Clear Linux OS uses today, tweaked to be more specific/accurate and to reflect consolidating all comments to the top of the file.

[ahkok]

That's fine.