add image to history page

[ultrasaurus]

adding timeline image to history page

• created assets/timeline.png (right now just a cropped version of 2018-10-12-timeline.png, but I figure we can use that URL for one that we keep up to date)
• added into about > how we came to be (page where we have text about the history)

[ultrasaurus]

related issue to add detail about the timeline: https://github.com/bridgefoundry/bridgefoundry.github.io/issues/138