Update README.md

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README.md

@@ -2,3 +2,30 @@ generic-syntax-expanders
 ========================
 
 A Racket package for creating macros with built-in support for defining sub-macros within them, similar to match expanders
+
+Sometimes complex Racket macros need a way for their syntax to be extended. The Racket match form is an example, new patterns can be created for match using define-match-expander. This racket package provides a form for defining a macro with the ability to define expanders for the macro. Consider this contrived example:
+
+(define-syntax call-each
+  (syntax-parser
+    [(_ f (expr ...))
+    #'(begin (f expr) ...)]))
+    
+(call-each displayln (1 2 3))
+
+If a user of this macro wished for a way to specify a range of numbers as the arguments, the user must define their own version of call-each with support for ranges that expands to using the original call-each. However, this racket package provides a form, define-syntax-with-expanders:
+
+(define-syntax-with-expanders call-each
+  (syntax-parser
+    [(_ f (expr ...))
+    #'(begin (f expr) ... )]))
+
+(define-call-each-expander foo
+  (syntax-parser
+    [(_ low:number high:number)
+    #`(#,@(range (syntax-e #'low) (syntax-e #'high)))]))
+    
+(call-each displayln (foo 1 4))
+
+The define-syntax-with-expanders form creates a define-call-each-expander form, which defines syntax transformers that are only used inside the body of a call-each form and expand before call-each does.
+
+You can install this package with `raco package install generic-syntax-expanders`.