docs(readme): explain how to install previous versions

Closes #191

README.md

@@ -4,5 +4,15 @@
 
 https://chrome.google.com/webstore/detail/ighdmehidhipcmcojjgiloacoafjmpfk
 
+## Installing Previous Versions
+
+1. Download and extract one of the files from the [Batarang releases page on GitHub](https://github.com/angular/angularjs-batarang/releases)
+1. Navigate to `chrome://chrome/extensions/` in Chrome
+1. If you've installed Batarang from the web store, disable or remove that version
+1. On the top right, check the checkbox for "Developer mode"
+1. Click "Load unpacked extension..."
+1. Select the directory where you extracted the extension
+1. Close and re-open any inspected tabs
+
 ## License
 MIT