Update README.md

Author: jon-batscha

README.md

@@ -1,3 +1,5 @@
+NOTE: we recently found a bug in the latest version, v4.0.0, that causes requests to `create_campaign` and `update_campaign` to error out. If you need to use those, we recommend using 3.0.0 for those until we have a patch in place. For folks using v3.0.0, you will need to refer to the API Reference Revision = `2023-06-15` (the previous API revision corresponding to SDK v3.0.0).
+
 # Klaviyo Python SDK
 
 - SDK version: 4.0.0
@@ -2916,4 +2918,4 @@ We stick to the following convention for parameters/arguments
 In the interest of making the SDK Pythonic, we made the following namespace changes *relative* to the language agnostic resources up top (API Docs, Guides, etc).
 
 - Resource names use Title + Snake Casing, (e.g. `Data_Privacy`)
-- function names and parameter names use snake case (e.g. `get_metrics`, and `profile_id`)
+- function names and parameter names use snake case (e.g. `get_metrics`, and `profile_id`)