add README.md, remove sha256.txt

Author: kaspar030

README.md

@@ -0,0 +1,20 @@
+# Overview
+
+This repository contains a copy of qDSA, cleaned up for easy inclusion as
+package for [RIOT](https://github.com/RIOT-OS/RIOT).
+
+See https://www.cs.ru.nl/~jrenes/ for more information.
+
+# How to use
+
+This repo is automatically pulled in from the main RIOT repository when using
+the qDSA package.
+
+# Original sources
+
+These are the sha256sums of the original archives as downloaded from
+the original source (https://www.cs.ru.nl/~jrenes/):
+
+bfebebf0be7482def3316af5ea58b2c39c1d0e3110fbe53fc4adad8f79295f77  arm-g2.tar.gz
+2c6f533c2b23a5dd92995a75baa50cf1163082eb3c79d071366ebeb5f53005ce  avr-g2.tar.gz
+0438e490753ce02b52f5345fff8e2a480dccae116914c364d84d65944d206dc5  cref-g2.tar.gz