After struggling twice through creating a GitHub App, I filed actions#89 and was encouraged to submit a pull request. This patch attempts to improve the documentation by:
1. Documenting the requirements for the GitHub app.
2. Providing URLs to make app creation easier like GitHub ARC docs do.

I am not certain whether everything I added is correct, so please take a look and confirm. Parts I am unsure of:
1. Is there any use case where users will want to create this app in their account, not in an organization? I included a link to cover that case, but I am not sure if it is necessary.
2. Is there any other customization I need to include in the URL query params?
3. Are the manual requirements I included correct?