docs: add a simulated "screenshot" of nano to the README

Suggested-by: Lars Wirzenius -- https://liw.fi/readme-review/

README

@@ -10,6 +10,29 @@ Purpose
     holding Shift, search-and-replace with regular expressions, and
     several other conveniences.
 
+Appearance
+
+    In rough ASCII graphics, this is what nano's screen looks like:
+
+   ____________________________________________________________________
+  |  GNU nano 5.8                  filename                  Modified  |
+   --------------------------------------------------------------------
+  | This is the text window, displaying the contents of a 'buffer',    |
+  | the contents of the file you are editing.                          |
+  |                                                                    |
+  | The top row of the screen is the 'title bar'; it shows nano's      |
+  | version, the name of the file, and whether you modified it.        |
+  | The two bottom rows display the most important shorcuts; in        |
+  | those lines ^ means Ctrl.  The third row from the bottom shows     |
+  | some feedback message, or gets replaced with a prompt bar when     |
+  | you tell nano to do something that requires extra input.           |
+  |                                                                    |
+   --------------------------------------------------------------------
+  |                       [ Some status message ]                      |
+  |^G Help       ^O Write Out  ^W Where Is   ^K Cut        ^T Execute  |
+  |^X Exit       ^R Read File  ^\ Replace    ^U Paste      ^J Justify  |
+   --------------------------------------------------------------------
+
 Origin
 
     The nano project was started in 1999 because of a few "problems"