[contrib] Describe how to untabify.

That's very snazzy (I didn't know about that repeat construct). But I don't think it is good here because that's not what people would actually type. I see what you mean about Markdown eating spaces, even in back quotes. But if you put it in triple back quotes, Markdown will keep the spaces: 
```
:1,$s/\t/    /g
```
Or, you could write something like "`:1,$s/\t/..../g` (where the dots represent 4 spaces)"