[design task] add social preview image #81
Description
Social preview images allow a nice image to show when the github repos and issues are shared, e.g. on twitter. see https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/customizing-your-repositorys-social-media-preview for more info about sizes - 1280*640px is best. Use the OBF logos in the /logos folder on this repo as a basis, but other than that, feel free to be creative
Specs:
- 1280*640px
- matches current OBF green colour theme and font
- required: svg and raster png/jpg files, other files (e.g. psd or .ai) optional.
you may also be interested in #82 at the same time.