Improve explanation in rules.md

I came across a bit of confusion on the last line of explanation, in Documentation → The Rules → [Anatomy of a typical Harp application](http://harpjs.com/docs/development/rules):

````
+- hello-world.jade  <-- must have an index.html or index.jade file
````

This line shows a **custom-named** file (`hello-world.jade`) while explaining that there must be a **fixed-name** `index` file (of which none exist in the `articles/` dir of this example).

Maybe it's too early in the morning for me, but this is not self-explanatory to me – is it required to have a html or hade file that's *exactly* named `index.ext` or not? Or is the actual requirement that there's *at least* one jade or html file in here? The messages are mixed regarding what's really required.

As I understand, the `index.jade` file in the `public/` dir *has* to be explicitly named `index`, right? If so, I think this possibly adds to the confusion regarding the above. (Again, this might be only me.)

Perhaps stating there must be `at least one *html or *jade file` might alleviate the risk for confusion?