Updated readme.

README.md

@@ -17,6 +17,12 @@ The Rust Cargo CNB will execute `cargo install`, which builds and installs your
 
 ## Usage
 
+In general, [you probably want the rust-cnb instead](https://github.com/dmikusa/rust-cnb/#tldr). If you want to use this particular CNB directly, the easiest way is via image.
+
+Run `pack build -b dmikusa/rust-cargo:<version> ...`. Alternatively, follow the instructions in the next section to build locally and `pack build -b rust-cargo-cnb_xxxx` (folder generated by the build script). The latter is useful when developing.
+
+## Building
+
 To package this buildpack for consumption:
 
 ```bash