Include docs on creating github app for example

[akerl]

Would it be possible to add code above the “example” section of the docs, or in a readme in that directory, that walks through the right github app settings for running the example?

As a first-time user of this library, I went to try the example. Running the example server was super straightforward, but I was left to guess at what options to set when creating my github app. Specifically, I had to infer what permissions to ask for, given the description of what the app would do, and I had to search in the code to find what “DefaultWebhookRoute” referred to.

[asvoboda]

I think we'll want to just use https://github.com/palantir/policy-bot#deployment along with some handy screenshots. Then we can probably remove the policy-bot section and just link here for all our bots.