docs: note DecodeError in the README response_model paragraph

One-line addition: response_model= decode failures raise
httpware.DecodeError (a ClientError subclass), so the standard
except httpware.ClientError catches them.

Author: lesnik512

README.md

@@ -49,7 +49,7 @@ with Client(base_url="https://example.test") as client:
     print(response.json())
 ```
 
-Typed decoding via `response_model=` works in both worlds — requires `pip install httpware[pydantic]`:
+Typed decoding via `response_model=` works in both worlds — requires `pip install httpware[pydantic]`. Decode failures (malformed body, schema mismatch) raise `httpware.DecodeError`, a `ClientError` subclass — so `except httpware.ClientError` covers them alongside transport and status errors.
 
 ```python
 from httpware import AsyncClient