docs: explain how to exempt apps from Gatekeeper

Author: claui

README.md

@@ -55,6 +55,7 @@ If your issue persists, search for it before opening a new one. If you find an o
   * [`source is not there` error](doc/reporting_bugs/a_cask_fails_to_install.md#source-is-not-there-error)
   * [`wrong number of arguments` error](doc/reporting_bugs/a_cask_fails_to_install.md#wrong-number-of-arguments-error)
   * [Unlisted reason](doc/reporting_bugs/a_cask_fails_to_install.md#unlisted-reason)
+* [The app can’t be opened because it is from an unidentified developer](doc/faq/the_app_cant_be_opened.md)
 * [`uninstall` wrongly reports cask as not installed](doc/reporting_bugs/uninstall_wrongly_reports_cask_as_not_installed.md)
 * [`Error: Unknown command: cask` error](doc/reporting_bugs/error_unknown_command_cask.md)
 * [My problem isn’t listed](https://github.com/Homebrew/homebrew-cask/issues/new?template=01_bug_report.md)

doc/faq/the_app_cant_be_opened.md

@@ -0,0 +1,17 @@
+# The app can’t be opened because it is from an unidentified developer
+
+Chances are that certain apps will give you a popup message like this:
+
+<img src="https://i.imgur.com/CnEEATG.png" width="532" alt="Gatekeeper message">
+
+This is a [security feature from Apple](https://support.apple.com/en-us/HT202491). The single most important thing to know is that **you can allow individual apps to be exempt from that feature.** This allows the app to run while the rest of the system remains under protection.
+
+**Always leave system-wide protection enabled,** and disable it only for specific apps as needed.
+
+If you are sure you want to trust the app, you can disable protection for that app by right-clicking its icon and choosing `Open`:
+
+<img src="https://i.imgur.com/69xc2WK.png" width="312" alt="Right-click the app and choose Open">
+
+Finally, click the `Open` button if you want macOS to permanently allow the app to run on this Mac. **Don’t do this unless you’re sure you trust the app.**
+
+<img src="https://i.imgur.com/xppa4Qv.png" width="532" alt="Gatekeeper message">