Add tone with buffer example (#117)

examples/tone-buffer.jl

@@ -0,0 +1,21 @@
+#=
+This example illustrates synthesizing a long tone in small pieces
+and routing it to the default audio output device using `write()`.
+=#
+
+using PortAudio: PortAudioStream, write
+
+stream = PortAudioStream(0, 1; warn_xruns=false)
+
+function play_tone(stream, freq::Real, duration::Real; buf_size::Int = 1024)
+    S = stream.sample_rate
+    current = 1
+    while current < duration*S
+        x = 0.7 * sin.(2π * (current .+ (1:buf_size)) * freq / S)
+        write(stream, x)
+        current += buf_size
+    end
+    nothing
+end
+
+play_tone(stream, 440, 2)