Include example with upload/download rates

Author: lovelydinosaur

docs/networking.md

@@ -129,6 +129,13 @@ with httpx.rate_limit("25MB/s"):
     ...
 ```
 
+Differing rate limits for upload and download speeds may be applied.
+
+```python
+with httpx.rate_limit(upload="10MB/s", download="25MB/s"):
+    ...
+```
+
 Supported data rates must be specified in bytes per second... `B/s`, `KB/s`, `MB/s`, `GB/s`, `TB/s`.
 
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