Try adding installer image to README

Author: matthew-brett

README.md

@@ -2,7 +2,7 @@
 
 Installers for the core Scientific Python packages for macOS, Windows, and Linux.
 
-<!-- img src="https://TODO.png" alt="Screenshot of installer running on macOS" width="450px" -->
+<img src="./images/sp_installer_mac.jpg" alt="Screenshot of installer running on macOS" width="450px">
 
 Please visit [the release page](https://github.com/scientific-python/installer/releases/latest) to download an installer for your OS. <!-- TODO: replace with link to tutorial if/when written. -->