Added Rust to supported languages for project() (#4750) [skip ci]

docs/markdown/Reference-manual.md

@@ -1109,8 +1109,8 @@ This function prints its argument to stdout prefixed with WARNING:.
 The first argument to this function must be a string defining the name
 of this project. It is followed by programming languages that the
 project uses. Supported values for languages are `c`, `cpp` (for
-`C++`), `d`, `objc`, `objcpp`, `fortran`, `java`, `cs` (for `C#`) and
+`C++`), `d`, `objc`, `objcpp`, `fortran`, `java`, `cs` (for `C#`),
-`vala`. In versions before `0.40.0` you must have at least one
+`vala` and `rust`. In versions before `0.40.0` you must have at least one
 language listed.
 
 The project name can be any string you want, it's not used for