Script to see Disk Usage

[d02d33pak]

{PR Description goes here}

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I have checked the following requirements before making this PR.

• [x] The script is marked as executable (chmod +x).
• [ ] I have added/updated the README entry for the script (if applicable).
• [ ] I have mentioned any outstanding dependencies and OS requirements for the script in the README.

[aviaryan]

Please resolve the conflicts. Also, if it can show other mounted drives as well, I think that would be better as people without Macs generally have multiple drives.

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Btw, just realized the df command (without any parameters) already does that. Since df is much shorter than diskUsage and is decently meaningful (describe filesystem), I think we don't need this PR. Therefore, I will close this now. Maybe we should have a section to include commands like df which are simple to use and easy to remember.