Add documentation

Author: novemberborn

README.md

@@ -1,3 +1,33 @@
 # @ava/get-port
 
 Experimental AVA plugin which works like [`get-port`](https://github.com/sindresorhus/get-port), but ensures the port is locked across all test files.
+
+Install this as a development dependency alongside AVA itself:
+
+```console
+npm install --save-dev @ava/get-port
+```
+
+Then make sure you've enabled the shared workers experiment:
+
+`ava.config.js`:
+
+```js
+export default {
+  nonSemVerExperiments: {
+    sharedWorkers: true
+  }
+};
+```
+
+## Usage
+
+```ts
+const {default: getPort} = require('@ava/get-port');
+
+test.before('get port', async t => {
+  t.context.port = await getPort();
+});
+```
+
+`getPort()` reserves a port, such that no other concurrently executing test file also using `getPort()` will select that same port. To ensure the port is available it then (briefly) listens. You can [pass options used when listening](https://nodejs.org/docs/latest/api/net.html#net_server_listen_options_callback). You can't pass `port` though.