Docs: add BUILD/run section for doc: CyberRT Python API: an example

cyber/python/README.md

-# Python3 Wrapper for Cyber RT
+# Cyber RT Python API : An Example
 
-## Usage
+This document is an example demonstrating how to use Cyber RT Python API
+to write your own Python3 programs. Please make sure you have built Apollo
+successfully.
 
-Make sure you have built Apollo successfully. Then in Python code:
+## Step 1: Write your own code.
+
+Save it as, say, `path/to/my_demo.py`.
 
 ```python3
+#!/usr/bin/env python3
+
 import sys
 
 from cyber.python.cyber_py3 import cyber
@@ -13,16 +19,52 @@ from cyber.python.cyber_py3 import cyber
 cyber.init()
 
 if not cyber.ok():
-    print('Well, something is wrong.')
+    print('Well, something went wrong.')
     sys.exit(1)
 
 # Do your job here.
 cyber.shutdown()
 ```
 
-Learn more usage from the [examples](cyber_py3/examples/) and
-[tests](cyber_py3/test/).
+## Step 2: Write Python rule for Bazel to build
+
+Edit `path/to/BUILD` file, add the followng section:
+
+```
+load("@rules_python//python:defs.bzl", "py_binary")
+
+# blablahblah...
+
+# Add your own section here
+py_binary(
+    name = "my_demo",
+    srcs = ["my_demo.py"],
+    deps = [
+        "//cyber/python/cyber_py3:cyber",
+    ],
+)
+```
+
+**Note**: Like C++, Python code is also managed by Bazel starting from Apollo 6.0.
+Please refer to [How to Build and Run Python Apps in Apollo](../../docs/howto/how_to_build_and_run_python_app.md) for more on that.
+
+## Step 3: Build and run the demo program
+
+Now you can run the following commands to build and run the demo program.
+
+```
+bazel build //path/to:my_demo
+./bazel-bin/path/to/my_demo
+```
+
+Or simply run
+
+```
+bazel run //path/to:my_demo
+```
+
+## More Examples ...
+
+Learn more Cyber RT Python examples under the [examples](cyber_py3/examples/) and
+[tests](cyber_py3/test/) directory.
 
-**Note:** Like C++ files, Python files are also managed by Bazel since Apollo 6.0. You can take
-[how_to_build_and_run_python_app.md](../../docs/howto/how_to_build_and_run_python_app.md)
-as reference.