Update README.md

Add rustup update reminder

README.md

@@ -242,7 +242,11 @@ sudo apt install zenoh-backend-influxdb
 
 > :warning: **WARNING** :warning: : Zenoh and its ecosystem are under active development. When you build from git, make sure you also build from git any other Zenoh repository you plan to use (e.g. binding, plugin, backend, etc.). It may happen that some changes in git are not compatible with the most recent packaged Zenoh release (e.g. deb, docker, pip). We put particular effort in mantaining compatibility between the various git repositories in the Zenoh project. 
 
-At first, install [Cargo and Rust](https://doc.rust-lang.org/cargo/getting-started/installation.html). 
+At first, install [Cargo and Rust](https://doc.rust-lang.org/cargo/getting-started/installation.html). If you already have the Rust toolchain installed, make sure it is up-to-date with:
+
+```bash
+$ rustup update
+```
 
 > :warning: **WARNING** :warning: : As Rust doesn't have a stable ABI, the backend library should be
 built with the exact same Rust version than `zenohd`, and using for `zenoh` dependency the same version (or commit number) than 'zenohd'.