Include our terms and use restrictions

[jimmackenzie]

We've updated our terms and added our Use Restrictions. In particular, we do not permit integrations from folks making and offering employee surveillance software.

[janejuenyang]

Thanks for putting this together, @jimmackenzie! Some thoughts:

1. I think reviewing the Terms and Use Restrictions should be step 1, even before registering
2. As you flagged earlier, we ought to update the branding page as well: https://github.com/basecamp/api/blob/master/sections/brand_guidelines.md. Looks like it’s a couple changes to: (1) include need to email a request to use the logos; and (2) an update of the BC3 logo.