Add API key usage note to Getting Started section (#601)

* Add API key usage note to Getting Started section

* Update free vs paid tier wording

Author: justinpolygon

README.md

@@ -24,6 +24,8 @@ pip install -U polygon-api-client
 
 To get started, please see the [Getting Started](https://polygon-api-client.readthedocs.io/en/latest/Getting-Started.html) section in our docs, view the [examples](./examples) directory for code snippets, or view the [blog post with videos](https://polygon.io/blog/polygon-io-with-python-for-stock-market-data/) to learn more.
 
+The free tier of our API comes with usage limitations, potentially leading to rate limit errors if these are exceeded. For uninterrupted access and to support larger data requirements, we recommend reviewing our [subscription plans](https://polygon.io/pricing), which are tailored for a wide range of needs from development to high-demand applications. Refer to our pricing page for detailed information on limits and features to ensure a seamless experience, especially for real-time data processing.
+
 ## REST API Client
 Import the RESTClient.
 ```python