[contrib] Describe how to untabify.

Apparently Ubuntu comes with two versions of vim: http://askubuntu.com/questions/483308/what-are-the-differences-between-vim-basic-and-vim-tiny 

I don't think I installed vim at all intentionally on my 14.04 installation so maybe it comes with vim.tiny enabled by default (I speculate that the version numbers are the same). Also, I don't think "retab" would be available in vi, it is a vim feature I think. You could mention retab and say if it doesn't work use `1,$s/\t/..../g` which will work everywhere.