Add Clean Up Old Homebrew Files as a mac til

README.md

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 ### Mac
 
+- [Clean Up Old Homebrew Files](mac/clean-up-old-homebrew-files.md)
 - [Default Screenshot Location](mac/default-screenshot-location.md)
 
 ### PostgreSQL

mac/clean-up-old-homebrew-files.md

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+# Clean Up Old Homebrew Files
+
+If you've been using [Homebrew](https://github.com/Homebrew/homebrew) for a
+while, you may have built up some cruft in the form old and outdated files.
+These will not be cleaned up automatically. You have do tell Homebrew to do
+so. This can be done with the following command.
+
+```bash
+$ brew cleanup
+```
+
+This command will report what files it cleans up as well as how much disk
+space it was able to clear.
+
+See `man brew` for more details.
+
+h/t Dorian Karter