Update included_software.md

Author: cecton

included_software.md

@@ -36,6 +36,9 @@ The specific patch versions included will depend on when the image was last buil
   * Not installed by default.
   * Supports any version that `swiftenv` can install later than `4.x`. Versions `4.x` and earlier will not work due to incompatible shared libraries.
   * 5.2 is installed if `Package.swift` is present and no version is specified with `SWIFT_VERSION` or `.swift-version`.
+* Rust
+  * Not installed by default.
+  * Supports any version that `rustup` can install.
 
 ### Tools