Change the readme.md to only include Rust build commands, now that we no longer build with corrosion

Author: NlightNFotis

src/libcprover-rust/readme.md

@@ -4,22 +4,21 @@ This folder contains the implementation of the Rust API of the CProver (CBMC) pr
 
 ## Building instructions
 
-There are two ways to build the project:
-
-1. As part of the CBMC project (using `cmake`) by building CBMC with the flag
-   `-DWITH_RUST_API=ON`. The outcome of this process is a compilation artefact
-   `libcprover-x.y.z.a` under the `<build>/lib` directory.
-2. By executing `cargo build` under this (`src/libcprover-rust`) directory.
-
-   For this to work, you need to supply two environment variables to the
-   project:
-
-   * `CBMC_LIB_DIR`, for selecting where the `libcprover-x.y.z.a` is located
-     (say, if you have downloaded a pre-packaged release which contains
-      the static library), and
-   * `CBMC_VERSION`, for selecting the version of the library to link against
-     (this is useful if you have multiple versions of the library in the same
-      location and you want to control which version you compile against).
+To build the Rust project you need the Rust language toolchain installed
+(you can install from [rustup.rs](https://rustup.rs)).
+
+With that instaled, you can execute `cargo build` under this (`src/libcprover-rust`)
+directory.
+
+For this to work, you need to supply two environment variables to the
+project:
+
+* `CBMC_LIB_DIR`, for selecting where the `libcprover-x.y.z.a` is located
+  (say, if you have downloaded a pre-packaged release which contains
+   the static library), and
+* `CBMC_VERSION`, for selecting the version of the library to link against
+  (this is useful if you have multiple versions of the library in the same
+   location and you want to control which version you compile against).
 
 As an example, a command sequence to build the API through `cargo` would look
 like this (assuming you're executing these instructions from the root level