add readme doc (#467)

* add readme doc

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doc/how-to-author-readme-file.md

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+# How to author readme files for CLI CodeGen
+
+It's straight forward to prepare the initial readme files for CLI CodeGen which would contains the configuration to customize CodeGen behavior if needed:
+
+## Step 1. Prepare readme.az.md
+
+Create readme.az.md in the same folder of readme.md and copy following content with the **{svcName}** replaced with proper name
+
+    ## AZ
+
+    These settings apply only when `--az` is specified on the command line.
+
+    ``` yaml $(az)
+    az:
+        extensions: {svcName}
+        namespace: azure.mgmt.{svcName}
+        package-name: azure-mgmt-{svcName}
+    az-output-folder: $(azure-cli-extension-folder)/src/{svcName}
+    python-sdk-output-folder: "$(az-output-folder)/azext_{svcName}/vendored_sdks/{svcName}"
+    # add additinal configuration here specific for Azure CLI
+    # refer to the faq.md for more details
+    ```
+
+## Step 2. Prepare readme.cli.md
+
+Create readme.cli.md in the same folder of readme.md with following content. No modification needed.
+
+    ``` yaml
+    # add any configuration here for all CLI languages
+    # refer to the faq.md for more details
+    ```
+
+## Step 3. Prepare readme.python.md
+
+If your service has released Python SDK, it's likely you already have the file. If not, you can create readme.python.md in the same folder of readme.md and copy following content with **{svcName}** replaced properly:
+
+    ## Python
+
+    ```yaml $(python)
+    python:
+        azure-arm: true
+        license-header: MICROSOFT_MIT_NO_VERSION
+        namespace: azure.mgmt.{svcName}
+        package-name: azure-mgmt-{svcName}
+        clear-output-folder: true
+    ```
+
+    ``` yaml $(python) && $(python-mode) == 'update'
+    python:
+        no-namespace-folders: true
+        output-folder: $(python-sdks-folder)/{svcName}/azure-mgmt-{svcName}/azure/mgmt/{svcName}
+    ```
+    ``` yaml $(python) && $(python-mode) == 'create'
+    python:
+        basic-setup-py: true
+        output-folder: $(python-sdks-folder)/{svcName}/azure-mgmt-{svcName}
+    ```
+
+## Step 4. Update readme.md
+
+Add following highlighted line in readme.md to enable CLI CodeGen in swagger PR:
+
+![sample image](images/author-readme1.png)
+
+If you have PR to **azure-rest-api-specs** repo, you should be able to see the CLI CodeGen triggered automatically for your swagger PR:
+![sample image](images/codegen-in-swagger-pr.png)
+
+## Ready to generate code
+All readme files should be ready now. You can refer to [here](how-to-generate.md) to generate your CLI code now if you want. 
+
+Feel free to reach to us at amecodegen@microsoft.com if you hit any issues or questions.