Update build instructions

Change-Id: I5c8152b2ab27be88cd0b7bdf6e16c242be4f06a7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fargo/+/377974
Reviewed-by: David Reveman <reveman@google.com>
diff --git a/README.md b/README.md
index fd98ece..22cef55 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,14 @@
 real device to copy build results and execute them. For the emulator there is a bit of
 [tricky set up](https://fuchsia.dev/fuchsia-src/getting_started#enabling_network) to do.
+### Building for use as a prebuilt
+From the directory you cloned into.
+    cargo build --release
+The resulting `fargo` binary will be in the target/release directory.
 ### Testing if Fargo is working
 Now to verify if Fargo is working correctly, try starting a fuchsia machine and executing a test.