Add LICENSE.txt attribution for thirdparty software

Author: dmbryson

LICENSE.txt

@@ -208,3 +208,18 @@
     portions of this Software are embedded into the binary product as a result,
     you may redistribute such product without providing attribution as would
     otherwise be required by Sections 4(a), 4(b) and 4(d) of the License.
+
+## Copyrights and Licenses for Third Party Software Distributed with swift-tools-support-async: ##
+
+The swift-tools-support-async software contains code written by third parties.
+Such software will have its own individual LICENSE file as part of its
+source or in the directory in which it appears.  This file will describe the
+copyrights, license, and restrictions which apply to that code.
+
+The following pieces of software have additional or alternate copyrights,
+licenses, and/or restrictions:
+
+Program             Directory
+-------             ---------
+BLAKE3              ThirdParty/BLAKE3
+