block shipscope alerts from notification center.

@mlocher It seems like numbers would be better than bullets in this PR, but they looked a little weird. Is there a good way to make an ordered list in these docs? I couldn't find any examples.



Provided steps to disable Shipscope alerts so they don't fill up the
Chrome Notification Center.
@mlocher You know what? You're right. "lives on" is better than "lives at". When I was reviewing, I was reading: 

```
lives on https://github.com/codeship/shipscop
```
which sounded odd, but this
```
lives on Github
```
is totally correct. I defer to your English skillz. :)