Add Bitcoin donation button

docs/index.md

@@ -6,7 +6,8 @@ Let us know about your ideas in #mailcow @ Freenode.
 
 [Servercow](https://www.servercow.de) - hosted mailcow, KVM based virtual servers, web-hosting and more.
 
-[![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=JWBSYHF4SMC68)
+[![Donate (PayPal)](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=JWBSYHF4SMC68)
+[![Donate (Bitcoin)](https://img.shields.io/badge/Donate-Bitcoin-blue.svg)](bitcoin:1E5rgzgA1sS3QH7r1ToWxRC3GEavfsGMrx)
 
 ## Get support