apps.test: document Synth.

src/apps/test/README.md

@@ -28,3 +28,38 @@ keys are defined:
 
 *Optional.* If this key is `true` packets from `rx` that do not match the next
 packet from `comparator` are ignored. The default is `false`.
+
+
+## Synth (apps.test.synth)
+
+The `Synth` app generates synthetic packets with Ethernet headers and
+alternating payload sizes. On each breath it fills each attached output link
+with new packets.
+
+    DIAGRAM: Synth
+    +-------+
+    |       |
+    |       *---- (any)
+    |       |
+    | Synth *---- (any)
+    |       |
+    |       *---- (any)
+    |       |
+    +-------+
+
+### Configuration
+
+The `Synth` app accepts a table as its configuration argument. The following
+keys are defined:
+
+— Key **src**
+
+— Key **dst**
+
+Source and destination MAC addresses in human readable from. The default is
+`"00:00:00:00:00:00"`.
+
+— Key **sizes**
+
+An array of numbers designating the packet payload sizes. The default is
+`{64}`.