Add a tip for macOS usage

Author: fxn

Diff for: README.md

@@ -10,7 +10,7 @@ You need to have Docker installed on your machine. You can find instructions on
 
 ## Installation
 
-Go to the [latest release](https://github.com/rails/rails-new/releases/latest) and download the executable for your platform (not the source code). For example, on M1 MacOS this would be `rails-new-aarch64-apple-darwin.tar.gz`. Once the download is complete, unzip the `.tar.gz` file, which will create the `rails-new` executable. Move the executable into your path so that it is ready to run from the command line.
+Go to the [latest release](https://github.com/rails/rails-new/releases/latest) and download the executable for your platform (not the source code). For example, on M1 macOS this would be `rails-new-aarch64-apple-darwin.tar.gz`. Once the download is complete, unzip the `.tar.gz` file, which will create the `rails-new` executable. Move the executable into your path so that it is ready to run from the command line.
 
 ## Usage
 
@@ -25,4 +25,10 @@ Or with options:
 rails-new myapp --main
 ```
 
+The first time you attempt to use the executable in macOS you may see a message like this:
+
+> "rails-new" can’t be opened because Apple cannot check it for malicious software
+
+In that case, please go to System Settings → Privacy & Security. You'll see a section mentioning "rails-new" with a button labeled "Allow Anyway" that you have to click.
+
 The list of available options is found in the [Rails guides](https://guides.rubyonrails.org/command_line.html#rails-new).