Include credit and link to Björn's repository.

Author: jpetrie

README.md

@@ -1,5 +1,9 @@
 Vim - the text editor - for Mac OS X
 
+This is the official GitHub repository for MacVim, the macOS port of [vim](https://github.com/vim/vim). It is based on
+[Björn Winckler's repository](https://github.com/b4winckler/macvim), which was official until he stepped down as the
+maintainer of the port.
+
 - MacVim homepage http://macvim-dev.github.io/macvim
 
 - Vim README https://github.com/macvim-dev/macvim/blob/master/README_vim.md