docs: replace non-working cargo install instructions (#2)

`cargo install zentinel-agent-js` fails: the crate is not published on crates.io.
Documents `cargo install --git`, which does work, plus the prebuilt release binaries.

Also points the bundle docs link at docs.zentinelproxy.io; the
zentinelproxy.io/docs/ URL 404s.

Author: raffaelschneider

README.md

@@ -23,12 +23,31 @@ zentinel bundle install js
 zentinel bundle install
 ```
 
-The bundle command downloads the correct binary for your platform and places it in the standard location. See the [bundle documentation](https://zentinelproxy.io/docs/deployment/bundle/) for details.
+The bundle command downloads the correct binary for your platform and places it in the standard location. See the [bundle documentation](https://docs.zentinelproxy.io/deployment/bundle/) for details.
 
 ### Using Cargo
 
+`zentinel-agent-js` is not published on crates.io, so `cargo install zentinel-agent-js` does not
+work. Install straight from the repository instead:
+
+```bash
+cargo install --git https://github.com/zentinelproxy/zentinel-agent-js
+```
+
+This builds and installs the `zentinel-js-agent` binary.
+
+### Prebuilt Binaries
+
+Each [release](https://github.com/zentinelproxy/zentinel-agent-js/releases) ships binaries
+for `linux-x86_64`, `linux-aarch64`, and `darwin-aarch64`:
+
 ```bash
-cargo install zentinel-agent-js
+VERSION=0.3.0
+PLATFORM=linux-x86_64   # or linux-aarch64, darwin-aarch64
+curl -fsSL -o zentinel-js-agent.tar.gz \
+  "https://github.com/zentinelproxy/zentinel-agent-js/releases/download/v${VERSION}/zentinel-js-agent-${VERSION}-${PLATFORM}.tar.gz"
+tar -xzf zentinel-js-agent.tar.gz
+sudo install -m 0755 zentinel-js-agent /usr/local/bin/
 ```
 
 ### From Source