Update README.rst with note about server certificate changes (#514)

Author: greg-db

README.rst

@@ -1,3 +1,5 @@
+**Important: Make sure you're using v12.0.2 or newer of this SDK by January 2026 for continued compatibility with the Dropbox API servers. Please refer to this blog post for more information: https://dropbox.tech/developers/api-server-certificate-changes**
+
 .. image:: https://cfl.dropboxstatic.com/static/images/sdk/python_banner.png
     :target: https://github.com/dropbox/dropbox-sdk-python
 
@@ -70,4 +72,4 @@ This SDK is distributed under the MIT license, please see `LICENSE`_ for more in
 .. _CONTRIBUTING.md: https://github.com/dropbox/dropbox-sdk-python/blob/main/CONTRIBUTING.md
 .. _Developer Console: https://dropbox.com/developers/apps
 .. _OAuth Guide: https://www.dropbox.com/lp/developers/reference/oauth-guide
-.. _`Dropbox Support`: https://www.dropbox.com/developers/contact
+.. _`Dropbox Support`: https://www.dropbox.com/developers/contact