It would be great if you could allow having a bitcoin address as donate link in the readme.txt file of a plugin. I set in the readme.txt file of my plugin (g-crossposting) the following code, which makes the donate link disappear on the plugin homepage:
Donate link: bitcoin:18dNV4mZyQMDdgd1ebe2Gm4f9MNZMLuNNb
And if you want to make it perfect, please show a 2D QR code of the given address when clicking on the donate link in case it is a bitcoin address :-)
The URI scheme for bitcoin addresses is defined here: https://en.bitcoin.it/wiki/URI_Scheme