rust: alloc: clarify what is the upstream version

It may be unclear for readers which upstream Rust version these files
are based on. They may be unaware that they are intended to match the
minimum (and only, so far) supported version of Rust in the kernel.

Thus clarify it.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/alloc/README.md

@@ -10,6 +10,9 @@ upstream. In general, only additions should be performed (e.g. new
 methods). Eventually, changes should make it into upstream so that,
 at some point, this fork can be dropped from the kernel tree.
 
+The Rust upstream version on top of which these files are based matches
+the output of `scripts/min-tool-version.sh rustc`.
+
 
 ## Rationale