Add a README with setup instructions

[larryhudson]

Here's an outline for the instructions:

• Clone the repo, set up a virtual environment, then install the dependencies
• Set up a webhook URL with Smee.io and get it running on your computer
• Set up a new GItHub app
  • under 'Webhook', set the webhook URL to the one you created with Smee.
  • generate a random string and then set that as the 'webhook secret'.
  • under 'Private keys', create a new private key and download it. Store the .pem file in the project directory.
• Duplicate .env.sample to .env and then replace the placeholder values:
  • GITHUB_WEBHOOK_SECRET - the 'webhook secret' string you generated above
  • GITHUB_APP_ID - App ID at the top of the app settings
  • GITHUB_PRIVATE_KEY_PATH - relative path to .pem file (e.g. if it is called 'private-key.pem', then this value will be 'private-key.pem'
  • GITHUB_APP_USER_NAME - the 'name' of your GitHub app, followed by '[bot]'
• Install the App - in the left hand sidebar of the app settings, click 'Install App' and then choose the repositories you want to enable it for.
• Run python main.py to start the Flask dev server, and then start Smee to allow the webhook requests to be forwarded to the Flask server.
• Try creating a new issue in a repo where the app is installed. It should react to the issue with the 'eyes' reaction, then create a PR trying to resolve the issue. Have a look at the Flask dev server logs if you run into issues.