Write a page for new contributors

In conversation with @ceboudreaux in
[IRC](https://botbot.me/freenode/gittip/msg/4258008/) and @heidigardner
on #1120, it came out that Gittip has many sources of information and
it's hard to get oriented as a new contributor. Here's a page that aims
to fix this problem.
Learn to count
Rework first paragraph of /for/contributors/

Per feedback from @heidigardner and @ceboudreaux.

 - "new kind of company" -> "new kind of organization"
 - change "share" to "take" to indicate method of distribution
 - add two links and move a third
Specify that it's the *email* newsletter
Add departments and call to action
Tighten up opening and closing

Per @cakey in [IRC](https://botbot.me/freenode/gittip/msg/4555032/).
Smile!
Be less formal

Per @cakey in [IRC](https://botbot.me/freenode/gittip/msg/4555267/)
Expand types of work to five

Add legal and accounting
Merge branch 'master' into for-contributors
Tighten up /for/contributors/ and link from about

I dropped the "types of work" verbiage for now. Let's get this out the
door and then revisit how best to get business process documentation on
there.
Be explicit about working in public
Point to security file from contributors page
Note that twitter is my preferred contact method
Extricate public info from "where work happens"

We don't "do work" on Twitter, Facebook, or Tumblr. Those are ways we
communicate with the general public, but not ways we communicate with
each other as work.
Tweak about page per @ceboudreaux

Aligned contact us right (?!), changed call to action under get
involved (clicking the link doesn't make you a contributor).
Merge pull request #1135 from gittip/for-contributors

Add a /for/contributors/ page