Delete PREINSTALL.md

[Quidstein]

Welcome to the Experimental Extensions repo! If this PR adds a new extension, thank you, and please make sure you follow the steps below! Otherwise, you can disregard this message.

• [ ] The new extension's id roughly follows the pattern [firebase-product]-[action]-[noun], like storage-resize-images
• [ ] The following match the extension's id:
  • [ ] The folder that contains the extension (my-new-extension/)
  • [ ] The name field of my-new-extension/functions/package.json
  • [ ] The name field of my-new-extension/extension.yaml
  • [ ] A new entry in the packages array of the lerna.json file in the repository's root
• [ ] I've added a generate-readme script to my-new-extension/functions/package.json that matches the other extensions in the repository
• [ ] I've run the following npm commands at the root of the repository:
  • [ ] npm install
  • [ ] npm run generate-readmes
  • [ ] npm run format

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.