Add a /for/contributors/ page

@ceboudreaux and @heidigardner have both pointed out that there are a lot of sources of information about Gittip, and as a new contributor it's hard to know where to begin. Here's a page in the new overloaded `/for/` namespace (see https://github.com/gittip/www.gittip.com/issues/949?source=cc#issuecomment-19628561; we use `/for/` for communities, and I'm now proposing that we also use `/for/` for high-level pages targeting specific user groups such as companies and nonprofits).

With this pull request and #1134 I'm trying to move more to a model where I don't make changes to master directly, but rather I submit pull requests. This is a best practice and also opens up the field to other contributors participating as equals rather than me exercising special privileges. Hopefully over time that helps with team building. :-)

Also with this pull request, I've hand-rolled a QA instance of the app at Heroku so that non-technical contributors can view the changes on this branch without having to stand up the app themselves. I think we can greatly tighten our feedback loop if we streamline that process (#1133). Anyway, you can see the rendered page in this pull request here:

http://gittip-qa537.herokuapp.com/for/contributors/