add contributing page

I'd like to add a contributing page to the website to help people that want to get involved.

Also, I have some advertisements in the page that benefit me directly (and the ev3dev community indirectly) and I want to make sure that this is OK with everyone before I post it.

Check it out and let me know what you think: http://dlech.github.io/ev3dev.github.io/contributing/
Typo: "Others Linux"