Add documentation for advisories licenses (#1761)

LICENSE.txt

@@ -1,7 +1,7 @@
-All code and data in the RustSec advisory database repository is dedicated to
-the public domain:
-
-https://creativecommons.org/publicdomain/zero/1.0/
+All code and data in the RustSec advisory database repository are dedicated to
+the public domain (except otherwise specified, see exception below), and are
+available under Creative Commons Zero 1.O Universal license (see
+LICENSES/CC0-1.0.txt for terms).
 
 By committing to this repository, you hereby waive all rights to the work
 worldwide under copyright law, including all related and neighboring rights, to
@@ -10,10 +10,20 @@ the extent allowed by law.
 You can copy, modify, distribute, and retransmit any information in this
 repository, even for commercial purposes, without asking permission.
 
-Additional content from GitHub Security Advisory ("GHSA") database
+----
 
-Additional content may be adapted from GHSA with attribution requirements, but
-with no additional clauses like copyleft.
+Exception: Additional content imported from the GitHub Security Advisory
+("GHSA") database.
 
-Any such license and attribution will be explicitly covered on an advisory by
-advisory basis directly within the applicable advisories.
+Additional content is adapted from GitHub Security Advisory, and is available
+under Creative Commons Attribution 4.0 International (see
+LICENSES/CC-BY-4.0.txt for terms).
+
+Any such license and attribution will be explicitly covered, respectively in
+the "license" and "url" (pointing to the advisory GHSA identifier prefixed by
+https://github.com/advisories) metadata fields, directly within the applicable
+advisories. As stated in the terms linked below, this fulfills the attribution
+requirement.
+
+https://docs.github.com/en/site-policy/github-terms/github-terms-for-additional-
+products-and-features#advisory-database