asyncio_slow: Add example on chaining coros using "yield from" from docs.

asyncio_slow/test_chain.py

@@ -0,0 +1,18 @@
+#https://docs.python.org/3.4/library/asyncio-task.html#example-chain-coroutines
+#import asyncio
+import asyncio_slow as asyncio
+
+@asyncio.coroutine
+def compute(x, y):
+    print("Compute %s + %s ..." % (x, y))
+    yield from asyncio.sleep(1.0)
+    return x + y
+
+@asyncio.coroutine
+def print_sum(x, y):
+    result = yield from compute(x, y)
+    print("%s + %s = %s" % (x, y, result))
+
+loop = asyncio.get_event_loop()
+loop.run_until_complete(print_sum(1, 2))
+loop.close()