These are the companies or individuals contributing a monthly amount to help sustain Neovim's development. See the Bountysource campaign for more details.
Bountysource does not yet have an UI for sponsors to provide their URL. Most URLs displayed in the list were obtained using the GitHub API and a heuristic to match the Bountysource slug (a sort of sponsor identifier) with the GitHub username, but this method was error prone and failed for many sponsors, including those that don't have a GitHub account.
If you are pledging $5/month or more and want to have your URL listed, please send a pull request to neovim.github.io with a new entry for your Bountysource slug in the js/sponsors-override.js file. If you don't have a GitHub account, you can write to the Neovim mailing list instead.
You can obtain your Bountysource slug with the following command:
curl --header 'Accept: application/vnd.bountysource+json; version=2' 'https://api.bountysource.com/supporters?team_slug=neovim&per_page=10000&page=1' 2> /dev/null | python -m json.tool | grep -C5 $NAME | grep slug
$NAME by your Bountysource display name as shown in the sponsor list.