Semaphore - Might need disclaimer

Not sure where the docs for this page are located, otherwise I would have just done a PR:
http://danger.systems/js/guides/getting_started.html

The semaphore integration works, but there's a catch. Semaphore won't do a PR build unless it's a fork. So anyone who owns the repo, and simply does branch PRs, will see this message:

I wrote to Semaphore and they responded with the following:
```
After a project has been added, Semaphore builds branches that are created within the repository.
Because of this, at the moment Semaphore doesn't support building pull requests that are opened
within the same repository. Opening a PR will trigger a build only if it was opened from a fork. For
more details please refer to this page.

Good news is that adding the option for building pull requests within repository is on our roadmap.
So, we'd really appreciate if you have few moments to describe your use-case. We don't have an
ETA yet, but we expect to deliver this feature during the first half of 2018.
```

I believe a caveat should be added to the docs (I personally moved danger-js to Travis for this reason). But then again, maybe danger JS should detect semaphore and be willing to skip the detection process?

I hope this helps ^

Start with the Getting Started page at http://danger.systems/js/guides/getting_started.html and locate its documentation source. Add a clear caveat that Semaphore only builds pull requests from forks, then verify the page explains the limitation and its effect on branch pull requests.

Written by the indexing model from the issue text.