Improve the README.md documentation of how to create a GitHub App

[grossag]

After struggling twice through creating a GitHub App, I filed https://github.com/actions/create-github-app-token/issues/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?

[grossag]

Hi @gr2m , are you able to take a look at this? My company is getting closer to deploying this app across multiple organizations and I'd like to get the README updated soon if at all possible. Thanks!

[gr2m]

@grossag can you get back to us regarding the open questions? No worries if you can't finish up the PR, just let us know

[grossag]

@grossag can you get back to us regarding the open questions? No worries if you can't finish up the PR, just let us know

Hi, sorry for the delay in responding. I'll work today on fixing this up based on your feedback. Back when I originally did the PR, I hadn't understood the breadth of how people use create-github-app-token. What you are saying makes sense though.