docs: legacy SDK link

Author: jho44

README.md

@@ -1,6 +1,8 @@
 # lob-python-sdk
 The Lob API is organized around REST. Our API is designed to have predictable, resource-oriented URLs and uses HTTP response codes to indicate any API errors.
 
+Looking for our [legacy Python SDK](https://github.com/lob/lob-python/tree/legacy_v4)?
+
 This Python package is automatically generated by the [OpenAPI Generator](https://openapi-generator.tech) project:
 
 - API version: 1.3.0
@@ -32,7 +34,7 @@ pip install git+https://github.com/GIT_USER_ID/GIT_REPO_ID.git
 
 Alternatively you can pull directly from pypi using:
 
-```sh 
+```sh
 pip install lob-python
 ```
 (you may need to run `pip` with root permission: `sudo pip install git+https://github.com/GIT_USER_ID/GIT_REPO_ID.git`)