Add Reductions as a clojure til.

clojure/reductions.md

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+# Reductions
+
+One of the core functions in Clojure is
+[`reduce`](https://clojuredocs.org/clojure.core/reduce). It allows you to
+build up some result based on applying a function to each value in a
+collection. Clojure provides a similar function that builds up a (lazy)
+sequence of intermediate values as it is performing a reduce.
+This function is
+[`reductions`](https://clojuredocs.org/clojure.core/reductions).
+
+Using `reduce` to sum a collection of integers looks like this
+
+```clojure
+> (reduce + 0 [1 2 3 4 5])
+=> 15
+```
+
+whereas `reductions` performing the same task will look like this
+
+```clojure
+> (reductions + 0 [1 2 3 4 5])
+=> (0 1 3 6 10 15)
+```
+
+h/t Josh Davey