Add License

[ericboucher]

This repository is missing a license, which may prevent others from using it if they don't know under which terms they can use this software.

FYI, I'm working with OVIO & UNICEF on a Digital Public Goods initiative, where we are identifying projects that fit the initial requirements, with plans to subsequently build and support the ecosystem for such projects. We're planning to include Airbel in the list of initial DPG candidates but we cannot add it until this repo includes an open license.

Thank you!