[fractal] doc: Document the MSRV

[Kévin Commaille <kcommaille src gnome org>]

commit a0561e010c533316c3b4f429cad6e24b9d2d0025
Author: Kévin Commaille <zecakeh tedomum fr>
Date:   Sun Apr 3 11:02:55 2022 +0200

    doc: Document the MSRV

 Cargo.toml | 1 +
 README.md  | 6 ++++++
 2 files changed, 7 insertions(+)
---
diff --git a/Cargo.toml b/Cargo.toml
index 6a0f28bc9..7dcaa6952 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -3,6 +3,7 @@ name = "fractal"
 version = "5.0.0-alpha"
 authors = ["Julian Sparber <julian sparber net>"]
 edition = "2018"
+rust-version = "1.58"
 
 [profile.dev.package."*"]
 opt-level = 3
diff --git a/README.md b/README.md
index c03a7cf0b..a5f89042d 100644
--- a/README.md
+++ b/README.md
@@ -59,6 +59,12 @@ flatpak install --user gnome-nightly org.gnome.Fractal.Devel
 
 ## Build Instructions
 
+### Minimum Rust version
+
+To build Fractal, Rust 1.58 is required. For development, you'll need to install the nightly
+toolchain to be able to run our pre-commit hook that validates the formatting and lints the Rust
+code.
+
 ### Flatpak
 
 Flatpak is the recommended way of building and installing Fractal.