As the entire core LXD team is employed by Canonical we don’t currently have a way to accept donations as a project. Donating to the Ubuntu Community fund would work though. It wouldn’t be usable directly by us but those funds are used to let community members organize and attend events, so could be used by someone who wants to talk about LXD at a conference for example.
https://discourse.ubuntu.com/t/ubuntu-donations-funding/29 has a bunch of details on this.
https://ubuntu.com/download/desktop/thank-you gets you to the donation page.