As Anki is run as a business, it is difficult to accept donations. Instead, we make available AnkiMobile to raise funds for development. You can purchase AnkiMobile on your Apple mobile device, and the proceeds from it go towards supporting Anki's development.
From time to time, people request other ways to contribute, such as by making donations via a site like Patreon, or via something like BitCoin. These would take time to set up and maintain, as we need to ensure no laws are being broken, and relevant taxes are being paid. At the moment we feel that the time is better spent working on Anki instead.
If you're unable to purchase the app for yourself or a friend, there are still other ways you can contribute. Answering questions over on the user forums is a big help, as every question the developers don't need to answer means more time that can be spent working on Anki. And if you don't feel comfortable answering questions, telling your friends about Anki helps out too.