Add note regarding versioning to homepage

Author: Dragos-Gabriel Danciulescu

source/mainnet/docs/index.rst

@@ -6,6 +6,18 @@ Concordium is a public blockchain platform designed for security, privacy, and s
 
 Explore our developer resources, including detailed documentation, tutorials, and a suite of tools to support your development journey.
 
+.. Note::
+
+   Currently, Rust toolchain versions up to ``1.81`` are supported. To set version ``1.81`` as the default, run these commands:
+
+   .. code-block:: console
+
+      $ rustup toolchain install 1.81
+      $ rustup default 1.81
+
+   The minimum supported rust version is currently version ``1.73``
+
+
 .. image:: ./images/frontpage.png
    :width: 400px
    :alt: frontpage image