Update README.md

Clarify that the translation between unstable and stable items is currently done in the `rustc_smir` crate.

Author: celinval

compiler/stable_mir/README.md

@@ -1,20 +1,28 @@
 This crate is currently developed in-tree together with the compiler.
 
 Our goal is to start publishing `stable_mir` into crates.io.
-Until then, users will use this as any other rustc crate, via extern crate.
+Until then, users will use this as any other rustc crate, by installing
+the rustup component `rustc-dev`, and declaring `stable-mir` as an external crate.
+
+See the StableMIR ["Getting Started"](https://rust-lang.github.io/project-stable-mir/getting-started.html)
+guide for more information.
 
 ## Stable MIR Design
 
 The stable-mir will follow a similar approach to proc-macro2. Its
 implementation is split between two main crates:
 
 - `stable_mir`: Public crate, to be published on crates.io, which will contain
-the stable data structure as well as calls to `rustc_smir` APIs and
-translation between stable and internal constructs.
+the stable data structure as well as calls to `rustc_smir` APIs. The
+translation between stable and internal constructs will also be done in this crate,
+however, this is currently implemented in the `rustc_smir` crate.[^translation].
 - `rustc_smir`: This crate implements the public APIs to the compiler.
 It is responsible for gathering all the information requested, and providing
 the data in its unstable form.
 
+[^translation]: This is currently implemented in the `rustc_smir` crate,
+but we are working to change that.
+
 I.e.,
 tools will depend on `stable_mir` crate,
 which will invoke the compiler using APIs defined in `rustc_smir`.