Add entry for GrandPerspective

[erwinbonsma]

Types of Changes

What types of changes does your PR introduce? Put an x in all boxes that apply

• New addition to list
• Fixing bug in existing item in list
• Removing item from list
• Changing structure (organization) of list

Proposed Changes

Add GrandPerspective to the list. This is an Open Source macOS application with a proven track record. It exists for more than fifteen years, is still actively maintained, has been downloaded more than two million times, and is rated highly on Sourceforge and the App Store.

PR Checklist

Put an x in the boxes once you've completed each step. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING guide
• I have explained why this PR is important
• I have added necessary documentation (if appropriate) - N/A

Further Comments

I added it to the section "System Related Tools" as DaisyDisk, a tool with a similar purpose, is also listed here. One might also argue that these tools should be listed under "File Organization Tools". If you would like to see it/them moved, let me know.