Document :prompt in README

README.asciidoc

@@ -810,6 +810,9 @@ Kakoune state:
 
 Some helper commands can be used to define composite commands:
 
+ * +:prompt <prompt> <register> <command>+: Prompt the user for a string, when
+     the user validates, store the result in given <register> and run <commmand>.
+     the -init <str> switch allows setting initial content. 
  * +:menu <label1> <commands1> <label2> <commands2>...+: display a menu using
      labels, the selected label's commands are executed.
      +menu+ can take a -auto-single argument, to automatically run commands