add companies blog post

Is this a good title? What's a better one?

Other things to be done:

- [x] add the email address, which one do we use? summer-of-code@gmail.org?
- [x] add the date for the campaign start
- [x] link to relevant external pages, e.g. Travis Foundation
- [x] link to the coaching companies page, if available, yet (we should try!)