docs: improved segment documentation (#1956)

packages/medusa-plugin-segment/READMD.md

@@ -0,0 +1,15 @@
+# medusa-plugin-segment
+
+Segment Plugin to add tracking to your Medusa server.
+
+Learn more about how you can use this plugin in the [documentaion](https://docs.medusajs.com/add-plugins/segment).
+
+## Options
+
+```js
+{
+  write_key: "<YOUR_SEGMENT_WRITE_KEY>"
+}
+```
+
+Where `<YOUR_SEGMENT_WRITE_KEY>` is the write key of the Segment Source you create for the Medusa server.