Added brief documentation with an example for file_stream

Added test to ensure `file_stream()` works in the test suite.

docs/sanic/response.md

@@ -60,6 +60,16 @@ async def index(request):
     return response.stream(streaming_fn, content_type='text/plain')
 ```
 
+## File Streaming
+For large files, a combination of File and Streaming above
+```python
+from sanic import response
+
+@app.route('/big_file.png')
+async def handle_request(request):
+    return await response.file_stream('/srv/www/whatever.png')
+```
+
 ## Redirect
 
 ```python

examples/try_everything.py

@@ -37,6 +37,10 @@ async def test_await(request):
 async def test_file(request):
     return await response.file(os.path.abspath("setup.py"))
 
+@app.route("/file_stream")
+async def test_file_stream(request):
+    return await response.file_stream(os.path.abspath("setup.py"),
+                                      chunk_size=1024)
 
 # ----------------------------------------------- #
 # Exceptions