show how to send multipart form data without files

As it is, the documentation hints at the possibility but it is not explicit. Hence it takes more time to figure it out on your own, especially for a less experienced developer.

docs/quickstart.md

@@ -169,7 +169,7 @@ Form encoded data can also include multiple values from a given key.
 }
 ```
 
-## Sending Multipart File Uploads
+## Sending Multipart File Uploads and/or Data
 
 You can also upload files, using HTTP multipart encoding:
 
@@ -221,6 +221,23 @@ If you need to include non-file data fields in the multipart form, use the `data
 }
 ```
 
+If you need to send non-file data only, you can do so by using a tuple
+of items for the field value without a filename:
+
+```pycon
+>>> files = {'some-field': (None, "field-value")}
+>>> r = httpx.post("https://httpbin.org/post", files=files)
+>>> print(r.text)
+{
+  ...
+  "files": {},
+  "form": {
+    "some-field": "field-value"
+  },
+  ...
+}
+```
+
 ## Sending JSON Encoded Data
 
 Form encoded data is okay if all you need is a simple key-value data structure.