Specify the GitHub bot requires "Write" access

[epologee]

If set to "Read", the build will fail with a message akin to the following:

Danger has failed this build. 
Found 1 error and I don't have write access to the PR set a PR status.

I do wonder if this is different for OSS projects, and whether getting_started.html.slim:63 should be changed as well:

In order to get the most out of Danger, we recommend giving her the ability to post comments in your code review. This is a regular GitHub account, depending on whether you are working on a private or public project you will want to give different levels of access to this bot. You are allowed to have [one bot per GitHub account][github_bots].

[orta]

Thanks - yeah, agree on this.

I think there should probably be something about the fact that the status API is optional, hrm, we should also use it to show a green when she passes too - hadn't thought of that