add clarification on what is stored and how

hostnames_allocator/README.md

@@ -57,6 +57,7 @@ See [src/ranged_number_allocator.rs](src/ranged_number_allocator.rs) for more de
 The asymptotic complexity above is theoretical; in practice, ideal trees don't actually work like that.
 In this implementation, Rust's [BTreeMap](https://doc.rust-lang.org/beta/std/collections/struct.BTreeMap.html) is used
 which is supposed to provide the same logarithmic asymptotic complexity in practice.
+Range starts are used as keys and range ends as values.
 
 Memory requirements are `O(log(number of ranges))` plus combined length of all host types.