Add CTA for adding new companies that links to GitHub

[doomspork]

Now that we've moved away from a form that submits listings for review, we should add a call to action on the homepage encouraging folks to add new companies or update dated listings via GitHub.

We had a similar ask last year when the process was different: https://github.com/beam-community/elixir-companies/issues/635. Might warrant a larger call out for how to maintain the list.