separate spec files documentation

Author: p1c2u

README.md

@@ -23,7 +23,16 @@ Validate spec:
 
 from openapi_spec_validator import validate_spec
 
-validate_spec(spec)
+validate_spec(spec_dict)
+```
+
+Add `spec_url` to validate spec with relative files:
+
+```python
+
+from openapi_spec_validator import validate_spec
+
+validate_spec(spec_dict,spec_url='file:///path/to/spec/openapi.yaml')
 ```
 
 You can also validate spec from url: