Add README with prerequisites to example app

Signed-off-by: Kate Goldenring <[email protected]>

Author: kate-goldenring

Cargo.lock

@@ -0,0 +1,21 @@
+# Hello World Command Spin App
+
+## Prerequisites
+
+1. Install [`cargo-component`](https://github.com/bytecodealliance/cargo-component) with a version that is compatible with WASI 0.2.0
+(at least `v0.7.0`).
+
+2. Install the command plugin from the releases or build building and installing from main:
+
+```sh
+cd ..
+cargo build --release
+spin pluginify install
+```
+
+## Running
+
+```sh
+$ spin build --up
+Hello, world!
+```