MEC added Diagonal Difference HackerRank

[basilshah]

Hey there!, Thanks for submitting a PR to this repo Please complete the following checklist for your PR to be approved. Mark the items by adding an x.
Example: - [ ] Mark me as completed
To mark: - [x] Mark me as completed

• [x] My Pull Request adheres to the Contribution guidelines.
• [x] I have added readme.md and solution files in the appropriate folder
• [x] None of the folder and file names I have added contain any invalid characters, leading digits or trailing whitespaces
• [x] The solution I have given is my own and not plagiarized in any form.
• [x] My solution passes all the testcases.
• [x] I have properly indented and documented my solution using comments.

If any of the above is unmarked, please make the necessary changes and then mark them so that you're PR can be approved and merged.

[mergify[bot]]

Awesome :thumbsup:, Thanks for contributing to Hack-CP-DSA by TLE MEC :smiley:!