doc/audioutils: Add rtttl-c documentation

Add documentation about rtttl-c, a simple RTTTL parsing library.

Signed-off-by: Jiri Vlasak <jvlasak@elektroline.cz>

Documentation/applications/audioutils/rtttl-c/index.rst

@@ -0,0 +1,24 @@
+==========================================
+``rtttl-c`` A simple RTTTL parsing library
+==========================================
+
+``rtttl-c`` is a simple library for parsing `Ring Tone Text Transfer Language`_
+(RTTTL).
+
+.. _Ring Tone Text Transfer Language: https://en.wikipedia.org/wiki/Ring_Tone_Text_Transfer_Language
+
+To use the ``rtttl-c``, first include it from the audioutils::
+
+        #include <audioutils/rtttl.h>
+
+then define what to do with a tone::
+
+        void
+        play_tone(struct rtttl_tone tone)
+        {
+                /* TODO */
+        }
+
+and finally play a RTTTL string::
+
+        rtttl_play("Jingle Bells:o=5,d=4,b=170,b=170:b,b,b,p,b,b,b,p,b,d6,g.,8a,2b.,8p,c6,c6,c6.,8c6,c6,b,b,8b,8b,b,a,a,b,2a,2d6", play_tone);