[README] Add macOS download package table (#8343)

- https://github.com/xamarin/xamarin-macios/issues/7462

README.md

@@ -23,6 +23,30 @@ If you are interested in fixing issues and contributing directly to the code bas
 - [Coding Guidelines](https://github.com/xamarin/xamarin-macios/wiki/How-to-Contribute#coding-guidelines)
 - [Submitting pull requests](https://github.com/xamarin/xamarin-macios/wiki/How-to-Contribute#pull-requests)
 
+## Downloads
+
+The preferred method for installing Xamarin.iOS and Mac is to use the Visual Studio installers ([Windows](https://docs.microsoft.com/en-us/xamarin/ios/get-started/installation/windows/?pivots=windows), [Mac](https://docs.microsoft.com/en-us/visualstudio/mac/installation?view=vsmac-2019)). 
+
+The team also [strongly recommends](https://docs.microsoft.com/en-us/xamarin/ios/troubleshooting/questions/old-version-xcode) using the latest Xamarin SDK and Xcode whenever possible.
+
+However, we provide links to older Xamarin.iOS and Mac packages for macOS downgrades and build machine configuration.
+
+
+| Platform        | Link |
+|-----------------|--------|
+| Xamarin.iOS d16.0 | [12.8.0.2](https://download.visualstudio.microsoft.com/download/pr/1fdd2124-10f8-40f3-9d9a-101c7953aa9d/5aa052be7fd1de213a6b7a8a96bea72b/xamarin.ios-12.8.0.2.pkg) |
+| Xamarin.Mac d16.0 | [5.8.0.0](https://download.visualstudio.microsoft.com/download/pr/a5bac460-0a54-4fea-9010-464b1ef8ee6f/9ccfa18c4ed093ee879c02a321965521/xamarin.mac-5.8.0.0.pkg) |
+| Xamarin.iOS d16.1 | [12.10.0.157](https://download.visualstudio.microsoft.com/download/pr/87d63fb1-c713-4c5f-b2c8-ae309a3b829c/91c05c2d5587f8f7efc5f7959f55eddf/xamarin.ios-12.10.0.157.pkg) |
+| Xamarin.Mac d16.1 | [5.10.0.157](https://download.visualstudio.microsoft.com/download/pr/fdaee266-06e3-46f0-bd67-5081279dd1cf/577697f6fbddd9542d1c862a8603d362/xamarin.mac-5.10.0.157.pkg) |
+| Xamarin.iOS d16.2 | [12.14.0.114](https://download.visualstudio.microsoft.com/download/pr/cce6ebe1-dd65-472c-9d01-d83561f341bd/a6ab5d414cdb3e85c9283a954e182b9d/xamarin.ios-12.14.0.114.pkg) |
+| Xamarin.Mac d16.2 | [5.14.0.114](https://download.visualstudio.microsoft.com/download/pr/9753ad10-d060-4427-880b-b4e0288878e1/140f9d84b62b3c57f00279f2b32360ce/xamarin.mac-5.14.0.114.pkg) |
+| Xamarin.iOS d16.3 | [13.6.0.12](https://download.visualstudio.microsoft.com/download/pr/ae8e9044-57e0-4500-bc58-6fe41d5f711d/71a1316dcca8c4cbcfb070aff9f738d2/xamarin.ios-13.6.0.12.pkg) |
+| Xamarin.Mac d16.3 | [6.6.0.12](https://download.visualstudio.microsoft.com/download/pr/c66cc733-8129-4551-8f59-4483f636931b/52194acdeb070158d49fe14cd1453062/xamarin.mac-6.6.0.12.pkg) |
+| Xamarin.iOS d16.4 | [13.10.0.21](https://download.visualstudio.microsoft.com/download/pr/16ae715b-ae37-47be-a5dc-c7abd236fbef/185ba3460c06e3fa2e7aa4609e63d57a/xamarin.ios-13.10.0.21.pkg) |
+| Xamarin.Mac d16.4 | [6.10.0.21](https://download.visualstudio.microsoft.com/download/pr/16ae715b-ae37-47be-a5dc-c7abd236fbef/90d9e8f056c51a45d9e649ec81b38126/xamarin.mac-6.10.0.21.pkg) |
+| Xamarin.iOS d16.5 | [13.16.0.13](https://download.visualstudio.microsoft.com/download/pr/fb168f8a-b44e-4582-8147-eefdf1562110/0a25988a5b9f502f4facd875c1b2072b/xamarin.ios-13.16.0.13.pkg) |
+| Xamarin.Mac d16.5 | [6.16.0.13](https://download.visualstudio.microsoft.com/download/pr/fb168f8a-b44e-4582-8147-eefdf1562110/30f09915995da752f30f432760e4ddbe/xamarin.mac-6.16.0.13.pkg) |
+
 ## Feedback
 
 - Ask a question on [Stack Overflow](https://stackoverflow.com/questions/tagged/xamarin.ios) or the [Xamarin Forums](https://forums.xamarin.com/)