added usage

README.md

@@ -2,6 +2,13 @@
 
 Rust support library for using [semaphore](https://github.com/appliedzkp/semaphore). It's mostly a Rust rewrite of [zk-kit](https://github.com/appliedzkp/zk-kit), but just focuses on semaphore (for now) and still covers a much smaller scope. It's using [ark-circom](https://github.com/gakonst/ark-circom) under the hood for generating the groth16 proofs.
 
+## Usage
+
+Add this line to your `cargo.toml`:
+```
+semaphore = { git = "https://github.com/worldcoin/semaphore-rs" }
+```
+
 ## Building semaphore circuits
 
 1. Check out submodule (if not done before already): `git submodule update --init --recursive`