setting content type for github webhook

[biw]

GitHub sends webhooks with an "application/x-www-form-urlencoded" by default, but this crashes the nuts server because it's excepting the content type to be "application/json". There's a dropdown to change the content type, but since there's no error message in the Heroku logs it was a little tricky to track down why the webhook wasn't working:

It'd be helpful to include this information in the docs so others don't get blocked by the same problem.

From GitbookIO/nuts#145

[biw]

This issue was fixed in #1