README: Add section on gptel-request

* README.org (** Using it your way): New section describing
gptel-request.

README.org

@@ -84,6 +84,17 @@ That's it. You can go back and edit previous prompts and responses if you want.
 
 The default mode is =markdown-mode= if available, else =text-mode=.  You can set =gptel-default-mode= to =org-mode= if desired.
 
+** Using it your way
+
+GPTel's default usage pattern is simple, and will stay this way: Read input in any buffer and insert the response below it.
+
+If you want custom behavior, such as
+- reading input from or output to the echo area,
+- or in pop-up windows,
+- sending the current line only, etc,
+
+GPTel provides a general =gptel-request= function that accepts a custom prompt and a callback to act on the response. You can use this to build custom workflows not supported by =gptel-send=.  See the documentation of =gptel-request=, and the [[https://github.com/karthink/gptel/wiki][wiki]] for examples.
+
 ** Why another ChatGPT client?
 
 Existing Emacs clients don't /reliably/ let me use it the simple way I can in the browser.  They will get better, but I wanted something for now.