Add brilliant.org to Education list closes #51

[nelsonic]

As discussed in #51 adding https://brilliant.org/ to the list.

Re-reviewed Contribution Guidelines to ensure changes abides.

Checklist:

• Company name: brilliant.org
• Company description - borrowed from their website
• Company location - SF + Remote
• GitHub link
• Sorted alphabetically
• Lines wrapped at 80 chars

Thanks.