Go to Support AnkiDroid from the hamburger menu. The first option in the menu says, “Donate” with a dollar sign next to it.
I think a monochrome heart icon would be better. Some organisations like Wikipedia already use 
for donations and this is also in line with what is often used in the changelog.
symbolises love, much better than what the dollar does (greed), and is also a more inclusive symbolism. It’s more widely understood.