Add instructions append

exercises/practice/binary-search/.docs/instructions.append.md

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+# Instructions append
+
+In Racket, this exercise uses sorted [vectors] rather than lists because vectors support constant-time access of its elements. Like several other Lisp tracks, when an item isn't present in the vector, the expectation is that the [Boolean literal `#f`][booleans] will be returned rather than raising an exception. 
+
+[vectors]: https://docs.racket-lang.org/guide/vectors.html
+[booleans]: https://docs.racket-lang.org/reference/booleans.html