add code snippet doc (Azure#27481)

* add code snippet doc

* Update

Author: xiangyan99

doc/dev/code_snippets.md

@@ -0,0 +1,51 @@
+# Code Snippets
+
+All code snippets in README should be up to date and runnable. And we want to have single source of truth hence we only need to maintain one copy. To achieve it, we want to have:
+
+- [Store samples in samples folder and setup the pipeline to run them to make sure no sample is broken](#create-samples)
+- [In README, refer the code snippet from the samples](#refer-samples)
+- [Run `python_snippet_updater.py` to keep them in sync](#python_snippet_updater-tool)
+
+## Create samples
+
+Samples need to store in the samples folder under each service. In each sample file, we use `# [START snippet_name]` & `# [END snippet_name]` to note a code snippet.
+
+A sample code snippet could be like:
+
+```python
+# [START asyncio]
+from azure.core.pipeline.transport import AsyncioRequestsTransport
+
+async with AsyncPipeline(AsyncioRequestsTransport(), policies=policies) as pipeline:
+    response = await pipeline.run(request)
+# [END asyncio]
+```
+
+https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/core/azure-core/samples/test_example_async.py#L63-L68
+
+## Refer samples
+
+Instead of copying the code snippet into README which is hard to maintain and validate, we add reference to the sample snippet in README. We use the annotation <!-- SNIPPET:file_name.snippet_name--> to refer the code snippet in README file.
+
+It is like:
+
+<!-- SNIPPET:test_example_async.asyncio -->
+```python
+from azure.core.pipeline.transport import AsyncioRequestsTransport
+
+async with AsyncPipeline(AsyncioRequestsTransport(), policies=policies) as pipeline:
+    response = await pipeline.run(request)
+```
+<!-- END SNIPPET -->
+
+## Run python_snippet_updater tool
+
+```powershell
+python <azure-sdk-for-python>/tools/azure-sdk-tools/ci_tools/snippet_update/python_snippet_updater.py <path_to_the_service>
+```
+
+The script scans the snippets in samples folder and auto replace the snippets in README with the one from samples folder.
+
+**NOTE: the snippets in README will be overwritten!**
+
+Now you are all set!

tools/azure-sdk-tools/ci_tools/snippet_update/python_snippet_updater.py

@@ -120,6 +120,6 @@ def update_snippet(file: str) -> None:
             except UnicodeDecodeError:
                 pass
     if not_up_to_date:
-        _LOGGER.error(f'Error: code snippets are out of sync. Please run Python PythonSnippetUpdater.py "{path}" to fix it.')
+        _LOGGER.error(f'Error: code snippets are out of sync. Please run Python python_snippet_updater.py "{path}" to fix it.')
         exit(1)
     _LOGGER.info(f"README.md under {path} is up to date.")