Add Expanding Macros as a clojure til.

clojure/expanding-macros.md

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+# Expanding Macros
+
+Macros are an important part of Clojure's syntax. They allow you to write
+cleaner, terser, more expressive code.
+Though sometimes you may want to inspect the
+clojure code that is actually produced by a particular macro. The
+`macroexpand` function allows for just this.
+
+For instance, if you have a snippet of code using the `->>` operator:
+
+```clojure
+(->> 4 (+ 1) (- 2) (* 3))
+```
+
+You can wrap that form with the `macroexpand` function to see the form that
+is ultimately evaluated:
+
+```clojure
+> (macroexpand (->> 4 (+ 1) (- 2) (* 3)))
+; (* 3 (- 2 (+ 1 4)))
+```
+
+It doesn't buy us much in a contrived example like this, but can prove
+useful for better understanding clojure and the more complex code we write.