add timeline profile howto

doc/fluid/howto/optimization/timeline.md

+## how to use timeline tool to do profile
+
+1. Add `with profiler.profiler(...)` to the main training loop. After run, the code will generate a profile record file `/tmp/profile`.
+
+	```python
+	with profiler.profiler('All', 'total', '/tmp/profile') as prof:
+	    for pass_id in range(pass_num):
+	        for batch_id, data in enumerate(train_reader()):
+	            exe.run(fluid.default_main_program(),
+	                    feed=feeder.feed(data),
+	                    fetch_list=[],
+	                    use_program_cache=True)
+	            ...
+	```
+
+1. Run `python paddle/tools/timeline.py` to process `/tmp/profile`, it will generate another
+file `/tmp/timeline` by default. You can change the path by cmd parameter, please take a look at
+[timeline.py](https://github.com/PaddlePaddle/Paddle/blob/develop/tools/timeline.py) for details.
+
+1. Open chrome and visit <chrome://tracing/>, use `load` button to load the generated `timeline` file.
+
+	![chrome tracing](./tracing.jpeg)
+
+1. The resulting timeline should be like:
+
+
+	![chrome timeline](./timeline.jpeg)