Add call for contributions on homepage

[sophiakc]

Context: Openfarm has been involving with open source contributions on the Github issues. This is a long list related to different topics, lacking visibility, priority order and reliable contribution.

Let's fix that! :)

From now the point of entry switch to Github Projects: https://github.com/openfarmcc/OpenFarm/projects/1
Every contributor of Openfarm is invited to look at this link first, as the issues are prioritized from "Backlog" through "To be done soon" to "Focus of the week".

Add in the footer of openfarm.cc:
Call for Developers (+url to https://github.com/openfarmcc/OpenFarm/projects/1)

[simonv3]

Nice! We should probably also add it to the README.md.

[sophiakc]

Related to: #908
The footer should have a clear onboarding for how visitors can contribute on the gardening or developer side

[simonv3]

This is also related to #912, where that call for volunteers is outlined.