docs: Add README

Author: erazemk

README.md

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+# Stack
+
+A tiny macOS menu bar app for managing tasks in a stack.
+The active task is always shown in the menu bar, and new tasks are added to the top of the stack (or optionally replace the current task).
+
+If you downloaded a release, macOS may quarantine it.
+To remove the quarantine attribute run `xattr -dr com.apple.quarantine Stack.app`.