Update 4.0.0-5.0.0-migration-guide.md (#434)

Author: mkozakov

4.0.0-5.0.0-migration-guide.md

@@ -26,6 +26,15 @@ for event in stream:
         print(event.text, end='')
 ```
 
+### Migrating deprecated `num_workers` Client constructor parameter
+
+The Client constructor accepts an `httpx_client` which can be configured to limit the maximum number of connections.
+
+```python
+limits = httpx.Limits(max_connections=10)
+cohere.Client(httpx_client=httpx.Client(limits=limits))
+```
+
 ### Removed functionality (subject to change)
 
 The following lists name the functions that are not in the new SDK and what their ongoing support status is.