[python] add contributing file (#5499)

Co-authored-by: iscai-msft <[email protected]>

packages/http-client-python/CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+## PR
+
+To observe the downstream effects of changes in this `@typespec/http-client-python` package on `@azure-tools/typespec-python`, we require a green PR to the [autorest.python](https://github.com/Azure/autorest.python) repo to ensure this.
+
+To make the downstream PR, follow these steps:
+
+1. Create your intended PR to the [microsoft/typespec](https://github.com/microsoft/typespec) repo for `@typespec/http-client-python`
+2. After the above CI passes, you get the url of a private package in CI.
+   ![alt text](image.png)
+   a. Click on the section that says `5 published; 1 consumed`, like in the above picture
+   b. Follow `Published artifacts -> build_artifacts_python -> packages -> typespec-http-client-python-x.x.x.tgz`.
+   c. Go to the right side, click the three dots, and click `Copy download url`.
+3. Create a PR in [autorest.python](https://github.com/Azure/autorest.python), updating the version of the `@typespec/http-client-python` in the `package.json` files to be the downloaded URL
+4. Run `pnpm install`, and follow that repo's [CONTRIBUTING.md](https://github.com/Azure/autorest.python/blob/main/CONTRIBUTING.md) for your second PR
+5. Once the PR to [autorest.python](https://github.com/Azure/autorest.python) passes, you can merge and release the original PR
+6. When the change to `@typespec/http-client-python` has been released, update your [autorest.python](https://github.com/Azure/autorest.python) repo to use the released version of the `@typespec/http-client-python` package