shell sort.cpp Add shell sort Algorithm

[pattarvarun]

For short term contributors: we understand that getting your commits well
defined like we require is a hard task and takes some learning. If you
look to help without wanting to contribute long term there's no need
for you to learn this. Just drop us a message and we'll take care of brushing
up your stuff for merge!

Fixes #270

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[sangamcse]

Again #273