Update readme.md (#19420)

Author: BigCat20196

specification/nginx/resource-manager/readme.md

@@ -39,3 +39,51 @@ These settings apply only when `--tag=package-2021-05-01-preview` is specified o
 input-file:
 - NGINX.NGINXPLUS/preview/2021-05-01-preview/swagger.json
 ```
+
+# Code Generation
+
+## Swagger to SDK
+
+This section describes what SDK should be generated by the automatic system.
+This is not used by Autorest itself.
+
+``` yaml $(swagger-to-sdk)
+swagger-to-sdk:
+  - repo: azure-sdk-for-net
+  - repo: azure-sdk-for-python-track2
+  - repo: azure-sdk-for-java
+  - repo: azure-sdk-for-go
+  - repo: azure-sdk-for-go-track2
+  - repo: azure-sdk-for-js
+  - repo: azure-sdk-for-node
+  - repo: azure-resource-manager-schemas
+  - repo: azure-powershell
+```
+
+## Go
+
+See configuration in [readme.go.md](./readme.go.md)
+
+## Python
+
+See configuration in [readme.python.md](./readme.python.md)
+
+## C#
+
+These settings apply only when `--csharp` is specified on the command line.
+Please also specify `--csharp-sdks-folder=<path to "SDKs" directory of your azure-sdk-for-net clone>`.
+
+``` yaml $(csharp)
+csharp:
+  azure-arm: true
+  payload-flattening-threshold: 1
+  license-header: MICROSOFT_MIT_NO_VERSION
+  namespace: Microsoft.Azure.Management.Advisor
+  output-folder: $(csharp-sdks-folder)/nginx/Microsoft.Azure.Management.Nginx/src/Generated
+  clear-output-folder: true
+```
+
+## Java
+
+See configuration in [readme.java.md](./readme.java.md)
+