doc [RUDOLPH-288] Add serverless signup to python SDK readme (#182)

### Summary

update readme so that it points to serverless as default url and add the
api key sign up url

README.md

@@ -25,7 +25,7 @@
 
 NOTE: This README is for the `0.26.0-beta` version. The current published SDK, `0.25.5` can be found [here](https://github.com/Unstructured-IO/unstructured-python-client/blob/v0.25.5/README.md).
 
-This is a Python client for the [Unstructured API](https://docs.unstructured.io/api-reference/api-services/overview).
+This is a Python client for the [Unstructured API](https://docs.unstructured.io/api-reference/api-services/saas-api-development-guide) and you can sign up for your API key on https://app.unstructured.io.
 
 Please refer to the [Unstructured docs](https://docs.unstructured.io/api-reference/api-services/sdk-python) for a full guide to using the client.