Merge pull request #793 from AdguardTeam/AGM-2802-update-installation.md-for-macos

AGM-2802 Added info about installation on older versions of macOS

Author: anastasiia-fedotova

docs/adguard-for-mac/installation.md

@@ -55,6 +55,12 @@ Now, you'll see a quick installation wizard where you'll be offered to configure
 
 Here we are, AdGuard is installed on your Mac!
 
+### Installation on older versions of macOS
+
+Users with older versions of macOS (Big Sur 11, Catalina 10.15, or earlier) won’t find a compatible app version on the AdGuard Ad Blocker website — these OS versions are no longer supported in the latest releases.
+
+However, you still can [download an older, compatible version](https://agrd.io/adguard_for_mac_v2-17)
+
 ## Uninstall
 
 ### Regular