Add advice about imports and name qualification to STYLE.md.

STYLE.md

@@ -7,6 +7,15 @@ style guidelines for that code are in the second section of this document.
 *ring* usually follows the [Rust Guidelines](https://aturon.github.io/), but
 there are some differences and *ring* adds additional guidelines.
 
+## Imports (`use`) and Qualification
+
+In general, import modules, not non-module items, e.g. `use core`, not
+`use core::mem::size_of_val`. This means that the uses of such functions must
+be qualified: `core::mem::size_of_val(x)`, not `size_fo_val(x)`. Exceptions may
+be made for things that are very annoying to qualify; for example, we usually
+`use super::input::*` or `use super::input::Input` because writing things like
+`input::Input` is highly annoying.
+
 ## Error checking
 
 Use `Result<T, ()>` as the return type for functions that may fail. Never use