[Tails-dev] roles description for contribute section

I've drafted a first batch of descriptions for the roles we identified
in the contribute section. Please have a look:

https://tails.boum.org/blueprint/contribute/

And here they are:

  - User: Every user can provide us with useful information or help
    others.
  - Donate: Donating speeds up the development of Tails.
  - Writer: Good writers can make Tails accessible to more people.
  - Translator: Translators can allow more people around the world to
    use Tails.
  - Speaker: Speakers can advocate Tails to all kinds of public.

I'd like to have the following tasks done before publishing our first
call for help in the news:

- role description (in progress)
- reorganize the second part of the page (ready for QA)
- explain our easy tasks (todo)

We're almost there!