Convert Readme of c/seq-mthreaded to Markdown and add license header

This allows us to remove the respective directory from .reuse/dep5.

I assigned the same copyright and license as for the actual sources
because the original submitter did not mention anything else.

Changes proposed by @PhilippWendler

.reuse/dep5

@@ -125,7 +125,6 @@ Files:
  c/recursive-with-pointer
  c/reducercommutativity
  c/regression
- c/seq-mthreaded
  c/seq-mthreaded-reduced
  c/signedintegeroverflow-regression
  c/sqlite

c/seq-mthreaded/README.txt → c/seq-mthreaded/README.md

@@ -1,3 +1,13 @@
+<!--
+This file is part of the SV-Benchmarks collection of verification tasks:
+https://github.com/sosy-lab/sv-benchmarks
+
+SPDX-FileCopyrightText: 2013 Carnegie Mellon University
+SPDX-FileCopyrightText: 2014-2020 The SV-Benchmarks Community
+
+SPDX-License-Identifier: LicenseRef-BSD-3-Clause-Attribution-CMU
+-->
+
 Contributed by: [HCCPS](http://www.sei.cmu.edu/cyber-physical/research/high-confidence) project
 
 The verification tasks should have the following convention: