pw_software_update: Add credit to TUF spec

No-Docs-Update-Reason: N/A

Change-Id: If3741283a3562b6bba3db1c92420ab60728bfe3b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/58201
Commit-Queue: Ali Zhang <alizhang@google.com>
Reviewed-by: David Rogers <davidrogers@google.com>
diff --git a/pw_software_update/tuf.proto b/pw_software_update/tuf.proto
index 6ad242c..f3c4e8f 100644
--- a/pw_software_update/tuf.proto
+++ b/pw_software_update/tuf.proto
@@ -11,6 +11,9 @@
 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 // License for the specific language governing permissions and limitations under
 // the License.
+//
+// Implementation of metadata formats specified in TUF Specification.
+// See https://theupdateframework.github.io/specification/latest/
 
 syntax = "proto3";