[Algo] Shell Sort [C++]

[joao-p-marques]

https://en.wikipedia.org/wiki/Shellsort

[joao-p-marques]

Can you assign this to me, please?

[sangamcse]

GitMate.io thinks possibly related issues are #250 ([Algo] Quick Sort [C++]), #222 ([Algo] Radix Sort [C++]), #258 ([Algo] Merge Sort [C++]), #20 ([Algo]: Add Bubble Sort in C++), and #62 ([Algo] Insertion Sort [C++]).