Refactor c/binary_search header

c/binary-search/src/binary_search.h

@@ -1 +1,12 @@
+#ifndef BINARY_SEARCH_H
+#define BINARY_SEARCH_H
+
+// binary_search expects:
+// val: an integer value to search
+// arr[]: an array of *sorted* integers
+// len: the length of ARR[]
+// If val is found, the function returns a pointer to an array index.
+// Otherwise, it returns NULL.
 int* binary_search(int val, int arr[], int len);
+
+#endif // BINARY_SEARCH_H