install-crate: update main readme

Author: Firestar99

README.md

@@ -1,6 +1,8 @@
 # cargo-gpu
 
-Command line tool for building Rust shaders using rust-gpu.
+An installation manager and a command line tool for [rust-gpu](https://github.com/Rust-GPU/rust-gpu/).
+
+For the command line tool, see below. To use the installation manager from your build script or other executables, see the [readme of `cargo-gpu-install`](crates/cargo-gpu-install/README.md).
 
 ## Getting Started