tests/driver_dac_dds: Replace hello file, add README

The previous file did not have a clear copyright situation; this one has
and it's described in the README.

tests/driver_dac_dds/README.md

@@ -0,0 +1,24 @@
+# About
+
+This is a manual test, and demo application for DAC DDS.
+
+# Usage
+
+This application will initialize the single default DAC DDS channel, and play
+sine, square, sawtooth waves or a greeting audio on it as requested on the command line.
+
+Typical usage:
+
+    > hello
+    > sine 440 2
+
+# Audio source and license
+
+hello.raw is an audio rendition of ESpeak saying "hello world". It was obtained
+from <https://commons.wikimedia.org/wiki/File:Hello_world_said_by_eSpeakNG.ogg>
+where it is annotated to be licensed under CC-BY-SA 4.0 international.
+
+It was converted and trimmed using:
+
+    $ fmpeg -i Hello_world_said_by_eSpeakNG.ogg -ar 8000 -f u8 -c:a pcm_u8 hello.raw
+    $ python3 -c "s = open('hello.raw', 'rb').read(); s = s.strip(b'\x80') + b'\x80'; open('hello.raw', 'wb').write(s)"