Add the Open Design event information to Ushahidi's GitBooks documentation

[Erioldoesdesign]

For publicity of the events mostly but good cohesion with the larger Ushahidi property.

Under the 'Hackathons and Events' heading

https://app.gitbook.com/@ushahidi/s/platform-developer-documentation/hackathon-and-events/installathon-may-2019