added images with the original code

Author: ramalho

references/sicp/instructors-manual/README.md

@@ -1,10 +1,12 @@
 # Concrete Metacircular Evaluator
 
-Code adapted from the "evaluator with concrete syntax" listed on pages 121-122
-of the "Instructor's Manual to accompany Structure and Interpretation of
-Computer Programs" by Julie Sussman with Harold Abelson and Gerald Jay Sussman.
+Code adapted from the "evaluator with concrete syntax" listed on pages 121-122 of
+["Instructor's Manual to accompany Structure and Interpretation of Computer Programs"](https://mitpress.mit.edu/books/instructors-manual-ta-structure-and-interpretation-computer-programs-second-edition)
+by Julie Sussman with Harold Abelson and Gerald Jay Sussman.
 
-Changes:
+Tested with [Racket](https://racket-lang.org/) 8.3.
+
+Changes from the original code:
 
 * function names changed to ALL-CAPS to avoid confusion with
   functions provided by Racket;
@@ -13,7 +15,7 @@ Changes:
 * APPLY rewritten to avoid the need for `primitive-procedure?`;
   the corresponding function in Racket is `primitive?` but it is not
   useful to us because `(primitive? +)` returns `#f`, so we can't use it
-  to distinguish user-defined procedures from the built-ins.
-
-Tested in Racket 8.3.
-
+  to distinguish user-defined procedures from those predefined by Racket;
+* added MAKE-BUILTINS to create an environment with predefined procedures;
+* added `concrete-test.rkt` with `rackunit` tests;
+* added `(provide ...)` to make `concrete.rkt` functions available for testing.