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
Whitespace cleanup for readability.
unnesting IDs to improve performance.
Reordering nested properties for readability.
Make one property declaration per line for readability.
Remove `! important` declarations

`! important` is not valid css and was being ignored in the browser.
`!important` may have been the original intent, but we recommend
using other techniques before this.
Set line-heights to 1em when they match the font-size to reduce redundancy.
Nesting elements to increase readability.
Merge pull request #1140 from strand/feature_responsive_readability

Feature responsive readability
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
Update balanced account links for new dashboard

Balanced has a new dashboard now, and we should use it (it has a
different URL).
Add departments and call to action
Up min threads for @jeresig
Bump version to 9.3.39
Bump version to 9.3.39-dev
Merge branch 'master' of github.com:gittip/www.gittip.com
Evasive action for #1151
Bump version to 9.3.40